MSc in Photonics PHOTONICSBCN

Universitat Politècnica de Catalunya (UPC) Universitat Autònoma de Barcelona (UAB) Universitat de Barcelona (UB) Institut de Ciències Fotòniques (ICFO)

http://www.photonicsbcn.eu

Master in Photonics

MASTER THESIS WORK

ZnO/PbS quantum dot heterojunction photovoltaics

Ariana Chouhan

Supervised by Prof. Gerasimos Konstantatos, (ICFO)

Presented on 6th September 2012

Registered at

ZnO/PbS quantum dot heterojunction photovoltaics

Ariana Chouhan Solution Processed Nano-Photovoltaics, Institut de Ciències Fotòniques, Av. Karl Freiderich Gauss 3, Castelldefels (Barcelona), 08068.

E-mail: ariana.chouhan@icfo.es

Abstract. Recent work based on photovoltaic heterojunction devices consisting of n-type ZnO and

p-type PbS is developed. We demonstrate that the PbS/metal electrode interface does not necessarily

form a Shottky barrier which is confirmed by a device ideality factor of 1.85 – consistent with a single

diode model. We believe planar heterojunction architectures impose an efficiency limit based on the

low contact area between interfacial layers and have thus developed recent work that introduced the

bulk nano-heterojunction – a facile method for mixing p and n type colloidal quantum dots together in

solution. Devices are based on n-type ZnO and p-type PbS which exhibit open circuit voltages as high

as 0.76 V.

Keywords: quantum dot photovoltaics, heterojunction, ZnO, PbS.

1. Introduction The use of Colloidal Quantum Dots (CQDs) in photovoltaics has been investigated rigorously since their discovery a decade ago. Today, thanks to widespread research and deeper understanding of the various physical processes that are unique to photovoltaic devices employing QDs, power conversion efficiencies are beginning to exceed 6% [1]. The bandgap of CQDs differs from that of their bulk counterparts due to quantum confinement of the exciton wavefunction in all three spatial dimensions. Such spatial confinement is only achieved on the length scale of the exciton Bohr radius (typically tens of nanometres) and as such, confinement effects do not become visible unless the QDs are smaller than this. The extent of the confinement is related to QD size – smaller dots result in blue-shifted (higher) bandgaps due to tighter confinement while relaxing the confinement by increasing the QD size reduces the bandgap towards that of the bulk material. CQDs are attractive for photovoltaics because their size is easily tuneable during synthesis – the band gap for PV materials is one of the most important properties in determining power conversion efficiency. Ideal single-junction solar cells should possess a bandgap in the near infrared (~1-1.5 eV) to absorb most of the high energy solar photons while still preserving open circuit voltage. Specifically, lead chalcogenide (PbX) devices are easily fabricated with band gaps ranging from 0.5 to 2.0 eV and are therefore well positioned to harness solar energy. Furthermore, it has been identified that lead sulphide (PbS) is an abundant and inexpensive semiconductor which exhibits a band gap of ~1-1.4 eV upon adding quantum confinement, placing it in the ideal range for photovoltaics [2].

1.1. Shottky diode Early solar cells made use of Shottky diodes employing a conductive oxide-QD-metal architecture where charge separation occurred between the non-ohmic metal-QD interface. Efficiencies of up to 3.3% have been reported for devices of this type [3]. However, the absence of a hole-blocking layer strongly limits the maximum open circuit voltage from such devices which have subsequently led to developments in device architecture.

1.2. P-N junction More recently, devices have been made using a p-n junction to separate charges – an n-type QD forms a p-n junction with a p-type QD in much the same way as p and n-type Silicon. The benefit over Shottky diodes is that open circuit voltages are generally higher due to the hole blocking properties of n-type QDs. Carrier extraction in thin film CQD solar cells works in the same way as for thicker, bulk solar cells like Silicon: The interface of p and n type materials creates a depletion region within the film which is the charge separation mechanism for excitons generated inside or near the depletion region. The biggest drawback in using QDs for light harvesting is the existence of surface defect states. Excitons generated in the active region must first be separated into holes and electrons before being transported to their respective electrodes. However, as the surface to volume ratio is much larger for QDs than for bulk materials, the effect of surface trap states is much more pronounced, leading to increased recombination and lower carrier mobility. As a result of poor mobility, the minority carrier diffusion length (proportional to the square root of mobility) is limited to ~ a few hundred nanometres. Therefore, minority carriers (electrons in PbS) generated more than the diffusion length away from the edge of the depletion region are more likely to recombine than be collected at the electrode. However, the typical absorption coefficients of these nanocrystals would require a film thickness of ~1 micron to absorb 100% of incoming AM 1.5G solar radiation at the excitonic peak of optimised QDs (~920 nm). This thickness mismatch has effectively placed a bottleneck on efficiencies for devices employing low mobility QDs which has prompted research into increasing the diffusion length to harvest more light without the recombination losses due to prohibitively thick films. 1.3. Ligand exchange By far the most researched and successful method has been through the use of ligand exchange. It is well known that the native Oleic Acid (OA) molecule which is used to effectively disperse the QDs in solution is very poor at transferring charges between adjacent QDs when in solid form due to the relatively long molecule length. It has been shown that the effectiveness of charge transfer between adjacent QDs has an exponential dependence on length [4]. Therefore, ligands should be as short as possible to shorten both the tunnel barrier that exists between adjacent dots as well as the physical distance between nanocrystals to produce denser films. To be effective, ligands should have functional heads to passivate the surface states of QDs thereby prolonging the carrier lifetime and boosting mobility. 1.4. Fabrication One of the main benefits of QD solar cells is their ease of fabrication – only a basic chemistry lab equipped with fume hood, glove box for sample storage and thermal evaporation chamber for contact deposition are required. Initially, a glass substrate coated with a transparent conducting metal oxide is usually the starting point for all devices. Then, depending on the architecture of the device, CQDs are spin cast to either form a p-n junction, or other charge separation mechanism. Metal contacts for the back electrode are deposited by thermal evaporation or sputtering to minimize damage to the QD film.

2. ZnO/PbS Heterojunction Most inorganic devices consist of planar interfacial layers in a stack. Our ZnO/PbS heterojunction device architecture is depicted in figure 1.

Figure 1. (a) Device schematic of a representative ITO/ZnO/PbS/Au/Ag heterojunction device and (b) corresponding

cross-sectional FIB image. The scale bar is 100 nm. The energy level diagram of the planar heterojunction device is shown in Figure 2.

Figure 2. (a) Flat band energy level diagram representing the ZnO/PbS heterojunction device with ITO cathode and

gold anode. All energies are given in eV with respect to vacuum and are taken from literature [5]. The device is

illuminated from the ZnO side. (b) Band bending under forward bias. EFn is the electron quasi-Fermi level (dashed line),

EFp is the hole quasi-Fermi level (dotted line).

2.1. Role of Zinc Oxide Zinc oxide has become very attractive for use in nano photovoltaics due its optical transparency (wide band gap), abundance, and ease of fabrication and processing. The response of ZnO to ultraviolet radiation is also well-known and is in fact a necessary property for use in solar cells. ZnO is initially weakly conductive and becomes a strong electron acceptor through a processes known as photo-doping [6]. On sustained exposure to UV light, bonds are broken between Zinc and Oxygen and the compound becomes n-type as Oxygen becomes free to accept and transport electrons. The optically transparent ZnO is advantageous when considering device architecture; the PbS film can either be placed on top of ITO or ZnO for a normal or ‘inverted’ structure. 2.2. Device Fabrication ZnO is synthesised according to literature methods and dispersed in a mixture of 95% chloroform and 5% methanol to improve dispersibility [7]. The resultant solution is diluted to 35 g/l and spin cast at 2000 rpm onto a chemically cleaned ITO covered glass substrate to achieve a thickness of ~100 nm. Oleic acid (OA) capped PbS with a mean diameter of 3.6 nm is synthesised via a slight variation on literature methods and dispersed in anhydrous toluene [8]. The nanoparticles are diluted to 35 g/l and spin cast in a sequential layer by layer fashion to achieve desired film thickness. For each layer, PbS is drop cast to evenly cover the

ITO

PbS

Au

Ag

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ZnO

ZnO

PbS 3.6 nm (1.3 eV)

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substrate and spin cast at 2000 rpm for 30 seconds. While spinning, two drops of 2% v/v 2-ethanedithiol in acetonitrile are applied, followed by a rinse in acetonitrile and toluene. The process is repeated to achieve the desired film thickness. Completed films are transferred to an N2 glovebox and stored overnight to allow remaining solvents to outgas. Au contacts are deposited by thermal evaporation at a base pressure of 1 x 10

-6

Torr and deposition rate of 0.05 nm s-1

to minimise damage to the PbS film. After 50 nm of Au is evaporated, 150 nm of Ag is deposited to reduce the chance of short circuits when performing J-V measurements. The device area is defined by the circular shadow mask used during evaporation which measures 3.14 mm

2. All

devices are characterised in air ambient conditions. 2.3. Device Characterisation A combination of physical, optical and electrical measurements provides a broad understanding of our device characteristics. 2.3.2. Current – Voltage. The most basic solar cell characteristics are the open circuit voltage (Voc), short

circuit current (Jsc), Fill Factor (FF) and overall power conversion efficiency (P) expressed as a percentage. These four parameters are simultaneously obtained through current-voltage measurements. Typically the device is connected using a purpose built probe station and voltage sweeps from 1 V to -1 V are performed to generate the typical J-V profile of solar cells. 2.3.3. External Quantum Efficiency. Spectrally resolved External Quantum Efficiency (EQE) measurements allow for further characterisation of our devices. For the ITO/ZnO/PbS/electrode architecture, EQE measurements provide useful insight into the shift of the excitonic peak of PbS due to the ligand exchange. 2.3.4 Internal Quantum Efficiency. Using the data from EQE and by measuring spectral reflectance from our devices, Internal Quantum Efficiency (IQE) can be calculated:

𝐼𝑄𝐸(𝜆) =𝐸𝑄𝐸(𝜆)

1 − 𝑅(𝜆) − 𝑇(𝜆)

where 𝑅(𝜆) and 𝑇(𝜆) are the spectrally resolved reflectance and transmittance of the device respectively. Due to the 200 nm thick metallic back electrode, 𝑇(𝜆) is zero for our devices. IQE is useful to understand how the device stack affects optical absorption. At certain wavelengths the distance between interfacial layers could affect light absorption by generating Fabry-Pérot resonances which may either be constructive or destructive. We also infer the effect of short diffusion length on our devices. The active PbS layer should not exceed the diffusion length or excessive recombination begins to hamper device performance. Thus, thin films are required to effectively harvest carriers. However IR absorption increases with thickness and as such, absorption is limited at longer wavelengths. IQE measurements provide a method to quantify IR absorption in our devices. 2.4. The effect of Schottky formation on the hole-extraction electrode

Bulovic et al. have reported the existence of a reverse bias Shottky barrier at the PbS/Au interface of their (identical) ZnO/PbS bilayer devices which are reported to detrimentally affect device performance [5]. Open circuit voltages of 0.41 V for devices comprising a 50 nm gold anode were reported which were ascribed to the presence of a Shottky barrier between PbS and Au when fabricated and tested in an oxygen-free environment. Here we demonstrate that no such barrier exists even for devices fabricated and tested in ambient conditions by fitting dark J-V data to the generalised single-diode Shockley equation

𝐽 =𝑅𝑝

𝑅𝑠+𝑅𝑝 {𝐽𝑠 [exp (

𝑞(𝑉−𝐽𝑅𝑆)

𝑛𝑘𝐵𝑇) − 1] +

𝑉

𝑅𝑝}

where Rp is the shunt resistance, Rs is the series resistance, Js is the reverse saturation current, and n is the diode ideality factor.

(1)

(2)

Figure 3. Dark current for ITO/ZnO/PbS/Au/Ag devices (circles) and corresponding fit to the generalised Shockley

equation (line). All parameters used for the model [Rs = 100 Ohm, Rp = 10 M Ohm, Js = 38 nA, n = 1.85] are determined

from dark J-V data which are representative of 5-6 devices on the same substrate.

A diode ideality factor of 1.85 indicates that the behaviour of these devices is indeed consistent with a single-diode model and confirms the absence of a Shottky barrier between PbS and Au. We ascribe the absence of a Shottky barrier to a slight energy offset between the PbS HOMO and Au work function which is enough to prevent the barrier from forming initially. While this energy offset is not visible in figure 2, it is known that different ligand exchange treatments of PbS nanoparticles result in various shifts in the excitonic peak as reported elsewhere [9]. However, our PbS nanoparticles are exposed to oxygen during both device fabrication and measurement which results in a certain level of oxidation, blue-shifting the position of the first excitonic peak. We believe that this shift equates to a slight deepening of the PbS HOMO, reducing the pre-existing energy offset with Au, enabling a Shottky barrier to form with time. We measure a first excitonic peak of 920 nm for OA-capped PbS in solution which, despite the effects of one hour of oxygen exposure, is red-shifted to 950 nm as a result of the ligand exchange and change of state of PbS into a densely packed film from solution. Blue-shifts of 0.7 eV have been reported over the course of one month of oxygen exposure for PbS thin films with almost half of which occurring in the first few hours [9]. Our measurement of the excitonic peak of 950 nm (through EQE measurement) one hour after oxygen exposure suggests that initially, a shallower HOMO level creates an energy offset at the PbS/Au interface. Despite brief exposure to oxygen during fabrication and transfer into/out of the evaporation chamber for characterisation, the existing energy offset at the time of dark J-V measurement is sufficient to prevent the Shottky barrier. This result also lends credence to the large scale fabrication of devices in ambient conditions which would reduce the already low cost of roll-to-roll manufacturing.

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Figure 4. IV of a typical ITO/ZnO/PbS/Au/Ag heterojunction after storage in air for three days. The roll-over in light

current observed at +0.6 V is due to the formation of a Shottky barrier as reported elsewhere [5]. Figures of merit:

[Voc = 0.47 ± 0.1 V, Jsc = 15.4 ± 1.0 mA cm-2

, FF = 55, p = 4.0 ± 0.4%].

Figure 5. EQE, IQE and spectral reflectance for the ITO/ZnO/PbS/Au/Ag bilayer from figure 4. The device is photo-

doped until the Jsc is saturated before measuring EQE without bias. Spectral reflectance is measured using an

integrating sphere. IQE is calculated using (1).

A prediction of the device Jsc can be obtained by integrating the product of the EQE and spectrally resolved solar irradiance which serves to validate the EQE measurement. For the bilayer device whose EQE is shown in figure 4, the predicted Jsc is 16.2 mA cm

-2 which is in close agreement with the measured value of 15.4 ±

1.0 mA cm-2

. While the device strongly absorbs visible light, a sharp onset is seen at 650 nm where reflectance increases by a factor of four and continues to rise into the infra-red. The PbS excitonic peak is clearly visible from both EQE and reflectance measurements at 950 nm. Despite the relative thinness of the device, over half of all photons absorbed by the PbS are converted into carriers at the excitonic peak. The drop in EQE at short wavelengths signifies the onset of glass absorption. The peak in reflectance at 600 nm is ascribed to Fabry-Pérot resonances in the device. From the glass/ITO

interface to the PbS/Au interface the device thickness is approximately /2, suggestive of a resonance at this wavelength.

3. ZnO/PbS bulk nano-heterojunction Fabrication of the BNH structure requires the extra step of mixing PbS and ZnO in solution before spin coating after the ZnO phase. This method allows for precise control over blend ratio and thickness, while leaving the overall fabrication procedure relatively unchanged. Due to the presence of transparent ZnO in the BNH phase, optical absorption is reduced compared to pure

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PbS (bilayers) however the physical thickness per LBL is unchanged which imposes further absorption limitations. Depending on the morphology of the BNH phase, carriers generated in the PbS will either be collected if a current percolation path exists to the electrodes or recombine if completely surrounded by ZnO. Similarly, isolated pockets of ZnO will accept electrons generated in PbS but cannot transport them to the ITO and will cause an increase in series resistance. We confirmed that ZnO and PbS are mixed on the nanoscale by transmission electron microscopy as illustrated in figure 6.

Figure 6. (a) Band bending in the BNH device. (b) Device schematic for the BNH structure (c.f. figure 1). Nanoscale

mixing ensures that the effective surface area of PbS:ZnO is increased which improves the effectiveness of exciton

separation. Also depicted are isolated pockets of PbS which cannot contribute to current generation due to the absence

of a current percolation path. Carriers generated in these regions recombine increasing series resistance. (c) TM

micrograph of the BNH film.

3.1. Enhanced open circuit voltage We note slight variations in figures of merit for blend devices between batches but consistently achieve higher open circuit voltages compared to the bilayer equivalent. The blend device with the highest observed

Voc has the following figures of merit: [Voc = 0.74 ± 0.1 V, Jsc = 6.1 ± 0.6 mA cm-2

, FF = 39, p = 1.8 ± 0.2%]. Changes in the Voc between batches reflect the differences between the quasi electron and hole levels in ZnO and PbS respectively due to variations in synthesis. Considering the energy difference between the ZnO

quasi electron and PbS quasi hole Fermi level of 0.75-0.78 eV, it is likely that 0.74 V reflects the maximum attainable Voc in the current device architecture. Achieving such open circuit voltages may be explained by the reduction of mid-gap defect states which trap carriers and detrimentally affect Voc. The effect on Voc of trap state density in almost identical devices to the PbS/ZnO bilayers presented here has been modelled by Ip et al. [10]. For a completely clean PbS mid-gap, i.e. a zero trap state density, the open circuit voltage was calculated to be approximately 0.72 V, closely reflecting our experimental results. We propose that ZnO inside the blend effectively passivates the

(b)

ZnO

PbS

(c)

(a)

remaining PbS traps that remain after the ligand exchange, leading to enhanced open circuit voltages.

ZnO in the blend also acts as a spacer, preventing the 20 meV red-shift of the excitonic peak observed in

pure PbS due to the ligand exchange process. Thus, 10% of the Voc enhancement compared to bilayers is due to the spacing effect of ZnO. This small enhancement comes at the cost of reduced short circuit current as the effective bandgap is now slightly wider than that of the bilayer for a given initial PbS size, reducing the portion of the solar spectrum that can be harnessed. 3.2. Enhanced current generation To compare the efficiency of charge extraction between bilayer and BNH devices we matched physical thickness and compared J-V characteristics.

Figure 7. J-V characteristics under dark and illumination for a BNH (a) and bilayer (b) device fabricated in the same

batch (different to the bilayer from figure 4). The bilayer consists of 250 nm of PbS; the BNH contains 190 nm of PbS

and ZnO mixed in a ratio of 1:1 and is capped with 60 nm of PbS. J-V data are averaged over several devices on the

same substrate and are taken after at least two minutes of illumination to ensure that the ZnO is sufficiently photo -doped.

As a result of isolated ZnO and PbS that are unable to contribute to charge extraction, series resistance of the BNH device increases to 1500 Ohm from 100 Ohm for the bilayer. Remarkably however, the short circuit current for the BNH is equal to that of the bilayer despite a significant reduction in optical density. This is suggestive of effective nanoscale intermixing of the two species, resulting in smaller average distances travelled by minority carriers and reduced recombination. 3.3. Device Optimisation Different blend ratios and thicknesses of the PbS capping layer were investigated to elucidate their respective roles. We find that the PbS capping layer is necessary as it provides a physical barrier between ZnO and the back electrode, which otherwise allows the band bending in the BNH phase due to ZnO to extend throughout the device during photo-doping which detrimentally affects device stability by lowering Voc faster than for capped devices. Fill factor and Jsc are considerably reduced for devices without a capping layer due to direct contact between ZnO and Au, resulting in charge recombination. Initial Voc does not depend on the presence of a PbS capping layer.

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Voc = 0.71 V, Jsc = 8.9 mA cm-2

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Voc = 0.55 V, Jsc = 8.8 mA cm-2

,

FF = 47, p = 2.3 ± 0.2%

(a) (b)

Figure 8. Capping layer studies for the BNH device. A sharp decline in measured Voc with illumination time for un-

capped devices is observed. The capping layer serves to block downward band banding caused by the presence of ZnO

in the BNH. The rate of Voc decline for capped BNH devices is consistent with bilayer devices indicating that the BNH

architecture does not intrinsically affect stability. Devices are different from those in figure 7.

The effects on series and shunt resistance, Jsc and FF of varying the blend ratio are plotted in figure 9. All blend ratios are prepared volumetrically. Figure 9. Blend ratio studies for the BNH layer on device characteristics. (a) Shunt and series resistance as a function of

blend ratio for a device consisting of ten BNH layers (200 nm) and two PbS capping layers (40 nm). (b) Jsc and FF as

a function of blend ratio for the same devices as in (a). Devices are different to those in figure 8.

We observe a decline in Jsc as PbS is added to the blend ratio and a sharp increase in FF, corresponding to the simultaneous reduction in series resistance and increase in shunt resistance. The reduction in Jsc with increasing PbS:ZnO blend ratios is ascribed to the differences in morphology that result from different blend ratios. For blends rich in PbS, ZnO may become more easily isolated and unable to transport carriers to the pure ZnO phase and vice versa for ZnO-rich blends. The highest measured Jsc occurs for a blend ratio close to parity, indicating that both ZnO and PbS are able to form more effective current percolation paths than for other blend ratios. Series resistance decreases as the blend ratio is taken away from parity towards a PbS-rich nano-composite. While the Jsc decrease may be attributable to increased pockets of isolated ZnO, a reduction in series resistance would not result from such a morphology change. During the ligand exchange for the BNH phase, it is possible that OA binds to ZnO after being displaced by EDT, rather than being washed away in the bilayer case. Electrically insulating OA molecules are undesirable and would increase series resistance if present in devices.

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BNH devices fabricated by Rath et al. which utilize n-type Bismuth Sulphide in place of ZnO demonstrate a very modest increase in series resistance for progressively PbS-rich blends, contrary to our results [11]. This is due to the fact that the Bi2S3 nanocrystalline network has proved to be an efficient electron transporter; thus we expect to achieve significant performance improvement in our ZnO/PbS heterostructures once we identify a process to allow for efficient electron transport along the percolation path of ZnO nanocrystals.

4. Conclusions We demonstrate that despite being fabricated and tested in air ambient conditions, ITO/ZnO/PbS/Au/Ag heterojunction devices exhibit high open circuit voltages of up to 0.56 V and power conversion efficiencies of 4.0%. Coupled with the fact that these devices are well modelled by the single diode equation, we demonstrate that degradation of PbS allows a Shottky barrier to form at the Au interface but is initially absent. Bulk nano-heterojunction devices from ZnO and PbS demonstrate higher open circuit voltages than their bilayer analogues while preserving short circuit current, despite being optically thinner. In particular, we achieved Voc in excess of 0.7 eV which is near the theoretical maximum limit for this type of junction and size of PbS QDs in the trap-free regime [10].

5. Developments Our attention is now focused on reducing series resistance for blend devices. We are currently running a series of FTIR spectroscopy experiments to determine whether OA remains in our devices after the ligand exchange process.

Acknowledgements The use of ‘we’ throughout reflects that without the greatly appreciated efforts and contributions from others, this work would not have been possible. The author wishes to thank the following for most helpful discussions and assistance throughout the development of this work: G. Konstantatos, A. Stavrinadis, M. Bernechea, A. Mihi, L. Martinez, A. K. Rath, F. P. Garcia de Arquer and F. Beck.

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[9] Zarghami M H, Liu Y, Gibbs M, Gebremichael E, Webster C and Law M 2010 p-Type PbSe and PbS Quantum Dot Solids Prepared with Short-Chain Acids and Diacids ACS Nano 4 2475-2485 [10] Ip H A, Thon S M, Hoogland S, Voznyy A, Zhitomirsky D, Debnath R, Levina L, Rollny L R, Carey G H, Fischer A et al 2012 Hybrid passivated colloidal quantum dot solids Nature nanotechnology 127 [11] Rath A K, Bernechea M, Martinez L, Pelayo Garcia de Arquer F, Osmond J and Konstantatos G 2012 Solution-processed inorganic bulk nano-heterojunctions and their application to solar cells Nature photonics 6 529-534