Vanadium oxide as new charge recombination blocking layer for high efficiency dye-sensitized solar cells

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Vanadium Oxide as new Charge Recombination Blocking Layer for High Efficiency DyeSensitized Solar Cells Hytham Elbohy, Amit Thapa, Prashant Poudel, Nirmal Adhikary, Swaminathan Venkatesan, Qiquan Qiao

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S2211-2855(14)20044-9 http://dx.doi.org/10.1016/j.nanoen.2014.09.008 NANOEN483

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Received date: 11 July 2014 Revised date: 18 August 2014 Accepted date: 6 September 2014 Cite this article as: Hytham Elbohy, Amit Thapa, Prashant Poudel, Nirmal Adhikary, Swaminathan Venkatesan, Qiquan Qiao, Vanadium Oxide as new Charge Recombination Blocking Layer for High Efficiency Dye-Sensitized Solar Cells, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2014.09.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Vanadium Oxide as new Charge Recombination Blocking Layer for High Efficiency Dye-Sensitized Solar Cells

Hytham Elbohy, Amit Thapa, Prashant Poudel, Nirmal Adhikary, Swaminathan Venkatesan, and Qiquan Qiao* Department of Electrical Engineering, Center for Advanced Photovoltaics, South Dakota State University, Brookings, South Dakota 57007, USA *[email protected]

Abstract Vanadium pentoxide (V2O5) was adopted as a novel blocking layer in dye-sensitized solar cells (DSCs), leading to a significant efficiency increase from 8.78% to 9.65%. The addition of V 2O5 layer to nanocrystalline (nc)-TiO2 increased peak external quantum efficiency (EQE) from ~ 80% to ~ 88-89%. Cyclic Voltammetry analysis indicated a positive shift of Fermi-level in case of TiO2/V2O5 based cells supported by an increase of its capacitance comparing to bare TiO 2 based cells. Electrochemical impedance spectroscopy (EIS) results exhibited a ~ 5 times higher charge recombination resistance (RCT) in V2O5 layer modified DSCs than conventional cells, which indicated that back charge transfer from TiO 2 to tri-iodide in the electrolyte was substantially suppressed. Transient photovoltage measurements on conventional and V 2O5 layer modified cells were conducted and their decays were fitted to calculate the electron recombination lifetime (τn), which increased by a factor of ~3 in V 2O5-based DSCs. This indicated that V2O5 significantly reduced the recombination rate at TiO2/electrolyte interface, further supporting that V2O5 functioned as a new effective surface passivation layer. Keywords Dye-Sensitized Solar Cells, Vanadium Oxide, and Charge Recombination Blocking Layer 1

1. Introduction

Dye-sensitized solar cells (DSCs) have been attracting great attention since it was invented in 1991.1-8 Device efficiencies beyond 12 % have been in reach 9. In DSCs, the mesoporous TiO2 is sensitized by a dye, which acts as light absorber. 10-14 The light illuminating on the dye excites electrons from ground to excited states. The excited dye molecules inject electrons into TiO2, which then diffuse through the mesoporous TiO 2 to the electrode and reach the counter-electrode through an external circuit. Redox electrolyte I -/I3- is used to reduce the oxidized dye and meanwhile the oxidized redox gets electrons from the counter-electrode. 15 The power-conversion efficiency of DSCs is strongly dependent upon the minimization of back charge transfer (recombination) losses at the TiO2/electrolyte interface.16 Such charge recombination leads to losses in both the short-circuit current density (Jsc) and open-circuit voltage (Voc), resulting in a decrease in overall energy conversion efficiency (η). Several studies on the back charge transfer were reported recently, 17-20 and some methods to suppress recombination reaction have been suggested. 16,

21-25

The back charge transfer (recombination) to

the electrolyte mainly occurs at two interfaces: FTO substrate/electrolyte and TiO 2 photoanode/electrolyte. Two main approaches have been used to reduce the back charge transfer from FTO to electrolyte at the FTO/electrolyte interface.22,

26

One is to use a thin TiO2 compact layer to

minimize the exposed FTO surface that is not covered by the nanoporous TiO 2 film. The other approach is to use a blocking material such as poly (phenylene oxide) (PPO) to passivate the FTO substrate using electrodeposition technique. 22,

26

At the TiO2/electrolyte interface, since the

dye molecules need to attach directly to the TiO 2, the blocking of mesoporous TiO 2 photoanode is basically a little more complicated. Two methods have also been used to decrease the back charge transfer recombination at the TiO2/electrolyte interface. The first is to coat the 2

mesoporous TiO2 film with a thin layer of a wide bandgap semiconductor that has a conduction band more negative than TiO2, while the second is to use a material having an electronicinsulating coating such as Nb2O5, Al2O3, ZrO2 and CaCO3 to form an energy barrier at the TiO2/electrolyte interface. This surface barrier reduces the back charge transfer recombination rate and thus increases device performance.16, 23, 24, 26, 27 In this work, we report a solution-processed and cost-effective vanadium (V) oxide (V2O5) as a novel charge recombination blocking layer for higher efficiency in DSCs. To the best of our knowledge, this is the first time for V2O5 to be used as charge recombination blocking layer. The new type of photoanode consists of mesoporous TiO 2 covered with a layer of V 2O5. The conduction band edge of V2O5 is significantly more negative than TiO2 conduction band edge, which indicates that V2O5 should function as effective barrier layers for electron recombination at the TiO2 photoanode/electrolyte interface. As a result, we have found that VOC, JSC and overall efficiency was been significantly improved after applying V2O5 to the TiO2.

2. Experimental section 2.1. Device fabrication Fluorine doped tin dioxide (FTO) substrates were cleaned with detergent, deionized (DI) water, acetone, and 2-propanol in an ultrasonic bath for 10 min each. The FTO substrates were treated by submerging in the TiCl 4 aqueous solution (40 mM) for 30 min at 70 oC. Then, a thin compact layer of TiO2 was spin coated onto the cleaned FTO substrates, followed by the deposition of a nanocrystalline TiO2 (Ti-Nanoxide HT/SP, Solaronix) via doctor blading. The film was kept in air for ~ 10 min and then sintered at 475 oC for ~45 min. A TiO2 light scattering layer was prepared by doctor blading a mixture of 80% Ti-Nanoxide R/SP (particle size > 100 nm) with 20% Ti-Nanoxide HT/SP TiO2 (particle size 8-10 nm) and sintered as above. And then a 3

40 mM aqueous solution of TiCl 4 was used to treat the photoanode again. Powder sample of V2O5 was purchased from Sigma Aldrich and used as received. V 2O5 was dissolved in 5 ml acetonitrile to prepare a solution with a V2O5: acetonitrile weight ratio of 1:50. This V2O5 solution was then stirred with a magnetic bar on a hot plate at 60 oC for 1 hour. After that, it was drop casted onto TiO2 main layer using micropipette and waited for 15 seconds before spin coated with a spin speed of 4000 rpm for 10 seconds and sintered at 475 oC. Then the film was cooled down to about 80 oC. The final photoanode was immersed in 0.25 mM N-719 dye solution in acetonitrile/valeronitrile (1:1 by volume) for 24 h to attach dye. N719 dye (Ruthenizer 535-bisTBA) was ordered from Solaronix. The dye-attached photoanodes were rinsed by acetonitrile to remove any extra dye and then dried by compressed nitrogen. The photoanode and counter electrode were then assembled and sealed using a thermoplastic sealant. 0.03 M I2, 0.60 M BMII, 0.10 M GuSCN, and 0.5 M tert-butylpyridine were dissolved in a mixture of acetonitrile and valeronitrile (85 : 15 by volume) to prepare the electrolyte, which was later injected into the cells. 28 For comparison, conventional DSCs without use of V2O5 were also fabricated as reference devices. The devices were illuminated with a mirror attached on their back under a solar simulator with an AM 1.5 filter at a light intensity of 100 mWcm -2. The device area was 0.13 cm2. The incident photon to electron conversion efficiency (IPCE) was recorded by a Newport M-QE Kit system made of a monochromator and a set of lenses. IPCE data were taken with the pace of 1 point per 5 nm. 2.2. Electrochemical impedance spectroscopy measurement Electrochemical impedance spectroscopy (EIS) measurements were conducted on both conventional and V2O5 treated DSCs in the dark on an Ametek Versastat 3200 potentiostat equipped with frequency analysis module (FDA). AC signal with 10 mV amplitude and 0.01 105 Hz frequency range was used. Forward bias close to open circuit voltage ≈ 0.8V was applied 4

with the photoanode negatively biased and counter electrode positively connected. The experimentally obtained impedance data were fitted by equivalent electrical model using EIS Spectrum Analyzer.

2.3. Cyclic Voltammetry measurements: The CV measurements were carried out in a solution using a solution of 10 mM lithium iodide (LiI) and 0.5 mM iodine (I2) as solutes with 0.1 mM tetrabutylammonium hexafluorophosphate) dissolved in acetonitrile as a supporting electrolyte. For comparison, the working electrode was TiO2 film deposited on FTO glass before and after deposition of V 2O5, while a platinum wire worked as a counter-electrode and Ag/AgCl as reference electrode.

2.4. Transient photo-voltage measurement DSCs were exposed to the white bias light from an array of light emitting diodes (LEDs) to generate photovoltage. A laser pulse from the OBB’s Model OL-4300 nitrogen laser (crisp pulse at 337 nm) was used to pump the model 1011 dye laser to generate an excitation source (pulse duration < 1 ns and repetition rate ~ 4 Hz).

The dye laser pulses having a specific

wavelength within the absorption spectral range of N719 illuminate the DSCs to produce transient photovoltage (TPVs) in the devices. TPVs were recorded using an Agilent MSO 07034B mixed oscilloscope (350 GHz, 2 GsaS-1) with 1 MΩ internal impedance, which were then fitted with mono-exponential decaying function to calculate carrier lifetime. 3. Results and discussion 3.1. J-V characteristics Figure 1 shows illuminated and dark current density-voltage (J-V) curves for both conventional and V2O5 modified non-masked cell. The comparison of photovoltaic parameters 5

between the conventional (TiO2/N719 dye/electrolyte) and V2O5 modified (TiO2/V2O5/N719 dye/electrolyte) cells was summarized in . Voc and Jsc increased from 0.8 V and 15.69 mA/cm 2 in conventional cells to 0.83 V and 16.97 mA/cm2 in the V2O5 modified devices, respectively. However, the fill factor (FF) showed a comparable value in the conventional (70%) cell and V2O5 modified (69%) devices. The V2O5 surface modification increased the device efficiency from 8.78% to 9.65%. The dark J-V curves of the conventional and V2O5 modified cells are also measured as shown in Figure 1b. Apparently, the dark J-V curve in the V 2O5 modified devices shows a larger turn-on voltage than that in conventional cells. This is consistent with the higher Voc observed in the V2O5 modified devices. Both the higher turn-on voltage in the dark and larger Voc in the illuminated J-V curves indicated that the addition of V2O5 led to the suppression of back charge transfer (recombination) from TiO2 to the I3- ions in the electrolyte. The enhancement of Voc is usually associated with the negative shift of conduction band edge which allows higher charge transfer from the excited state of the dye to the conduction band of the TiO 2 leading to lower charge back recombination rate to the dye and electrolyte. Figure 2 shows the energy band diagram of DSCs before (Figure 2a) and after (Figure 2b) adding V2O5 showing its role to suppress the electron back transfer (recombination) reactions indicated by red arrows. As shown in Figure 2b, the addition of a thin layer of V2O5 can form an energy barrier and block the back charge transfer from TiO 2 to the I3- ions in the electrolyte. V2O5 typically requires relatively specific conditions to be dissolved in acetonitrile such as stirring for long time at a temperature higher than room temperature otherwise it is just partially dissolve. Also, after V 2O5 was deposited, the film was annealed together with TiO2 at 475 oC to improve the crystallinity of film. However, there is possibility that V2O5 could partially redissolve in electrolyte at a very long time range since solar devices during operation can reach a temperature of the order of 80 oC, which may accelerate the dissolution of V2O5. 6

3.2. External quantum efficiency Incident photon-to-electron conversion efficiency (IPCE) as a function of wavelength is measured to evaluate the photoresponse of the conventional and V 2O5 surface-modified DSCs. Figure shows the IPCE action spectra for both cells. The maximum IPCE was improved from ~80 % in the conventional cell to ~ 88-89% for V 2O5-surface-modified DSCs. This implies that V2O5 can act as an effective blocking layer at the TiO 2/dye/electrolyte interface and thus suppress the back charge transfer (recombination) by passivating the TiO2 surface, improving device energy conversion efficiency from 8.78% to 9.65% in cell efficiency. To validate our J-V measurements, we estimated the Jsc by integrating the IPCE using equation below: 29 ఒ

‫ܬ‬௦௖ = ‫׬‬ఒ భ ݁ߟூ௉஼ா (ߣ)ܰ௣௛ (ߣ)݀ߣ బ

(1)

where ηIPCE(λ) and Nph(λ) are the IPCE and photon flux density at wavelength λ, respectively. Using eq. 1, the integration of the measured IPCE spectral response with the AM 1.5 solar spectrum (100 mW·cm-2) results in an estimated Jsc of 15.7 mA·cm-2 for the conventional DSCs and 17.3 mA·cm-2 for V2O5 modified cells, respectively. These calculated Jsc values were very close to the measured Jsc values (15.69 mA/cm2 for conventional cells and 16.97 mA/cm2 for V2O5 modified devices) from the J-V curves, showing that the device J-V testing is accurate.

3.3. Cyclic Voltammetry (CV) To understand more about the effect of adding the V 2O5 to TiO2, CVs of bare-TiO2 and TiO2/ V2O5 composite films immersed in the liquid electrolyte are shown in Figure 4. 7

The charge transfer process is typically related with back electron leakage to the electrolyte, which can be seen in the CV at low scan, 5 mV/sec in our ecperiment, rates and at intermediate return potentials.1, 30, 31 By calculating the capacitance, indicated by the current width of the CV curve in figure 4, it was found that the capacitance value is almost constant in the range of applied potential and has an average value of 7 × 10-6 F/Cm2 for TiO2/ V2O5 composite which was found to be almost 5 times higher than that of 1.55 × 10-6 F/Cm2 for bare-TiO2. This indicated that the Fermi level Efn is more close to the conduction band in TiO 2/V2O5 than TiO2 only, which is consistent with the slight increase the open circuit votage (VOC). 3.4. Electrochemical impedance spectroscopy (EIS) To further clarify the effects of V2O5 on the Voc, electrochemical impedance spectroscopy (EIS) was used to elucidate the electronic and ionic transporting processes in DSCs. EIS results can provide valuable information for the understanding of photovoltaic parameters (Jsc, Voc, FF, and η) in DSCs. The enhancement of Voc is usually associated with the negative shift of conduction band edge or the suppression of charge recombination. The value of Voc is determined by the potential difference between the Fermi level of TiO 2 and the chemical potential of the redox species (Ered) in the electrolyte, which can be described as the following equation:32 ܸ௢௖ = ߝ௥௘ௗ௢௫ − ߝ௖ −

ఊ௞ಳ ் ௘

ே

ln ( ௡೐)

(2)

Here γ is characteristic constant of TiO2 tailing states, kB is the Boltzmann constant, T is temperature, e is the elementary charge, and Ne is the effective density of states at the TiO 2 conduction band edge. When the DSCs is tested under the dark and forward bias, electrons are injected from FTO into TiO2 nanoparticles. Then the TiO2 is charged by electrons; simultaneously a portion of the injected electrons are lost by charge transfer from TiO 2 to I3- in the electrolyte. The EIS 8

Nyquist plots were measured bias of 0.6, 0.65, 0.7 and 0.75 V for both the conventional cell and V2O5 modified cell, respectively, as shown in Figure 5. The equivalent circuit for the DSCs is shown in Figure 5a, which was used to fit the experimental data of both the conventional and V2O5 modified cells. Rs represents the series resistance accounting for transport resistance through the FTO/TiO2 photo-anode / electrolyte interface. Cµ and RCT are the capacitance at TiO2/electrolyte interface and charge transfer (recombination) resistance from TiO 2 to the electrolyte at the dye-sensitized TiO2/electrolyte interface, respectively. CPt and RPt are the capacitance and charge transfer resistance at the Pt/electrolyte interface, respectively. The larger semicircle at lower frequencies represents the interfacial charge transfer resistance RCT at the dyed-TiO2/electrolyte interface. Since larger RCT value means higher charge recombination resistance between TiO2 and the redox electrolyte, RCT value for V2O5-based cells is much higher than that of conventional cells. This suggests that the V 2O5-based cells have a much lower recombination rate from TiO2 to electrolyte. For both conventional and V2O5 surface modified cell structures, as the forward bias potential changed from 0.6 to 0.75 V, the semicircles originated from the charge recombination at the TiO2/electrolyte interface became smaller. However at each forward bias, the RCT values of V2O5-based cells are larger than that of the conventional cells and this increase due to lower recombination rate at TiO 2/electrolyte interface which is consistent with the idea of charge recombination blocking layer. 33 The smaller RCT value indicates that the back electron transfer (recombination) from the TiO 2 to the electrolyte occurs more easily in the conventional DSCs, thus resulting in lower Voc. The chemical capacitance values (Cµ) have been measured from the fitting of Nequest plots and shown in table 3. the of V2O5-based cells has significantly higher capacitance values in the applied voltage range comparing to the bare TiO 2 and these values increases with increasing the applied voltage. These result were supported by the results obtained from the cyclic 9

voltammetry measurements even it was in different applied voltage range but have the same trend and support the idea of charge recombination blocking layer role of the V 2O5 inside the nano-crystalline TiO2. Also, we determined the electron recombination lifetime (τn) for both bare-TiO2 and TiO2/V2O5 composites which represented by the reciprocal value of the frequency of semi-circle peak.34, 35 The data values were calculated and plotted as shown in 6. The V2O5 surface modified cells exhibited a much longer electron lifetime (τn) than the conventional devices at all different biases, indicating the electron recombination was effectively suppressed at TiO 2/electrolyte interface by coating V2O5 onto TiO2 photoanode. As the voltage increases, the probability for electrons to recombine at TiO2/electrolyte interface is increased and RCT values decreased, which is consistent with that the time needed for electron to recombine (τn) is getting shorter (Figure 6). This phenomena were also supported by other reports.33

3.5. Transient photovoltage (TPV) measurement Figure 7 shows the transient photovoltage for both conventional and V 2O5-based cells. The longer electron recombination lifetime (τn) in the V2O5 treated DSCs than that in the conventional cells found in the EIS measurement was further confirmed by transient photovoltage measurement. As shown in Figure 7, the photovoltage decay at open circuit condition for the V2O5 modified cells exhibited a much longer τn (100 msec), more than three times of that (30 msec) for conventional cells. This further supports our previous analysis that spin coating V2O5 onto the TiO2 photoanode can significantly suppress the back electron transfer (recombination)

from

TiO2

to

the

electrolyte.

10

4. Conclusion In summary, V2O5 was used as novel electron recombination blocking layer in dye sensitized solar cells. The maximum IPCE was improved from about ~80 % for the conventional cell to up to ~88-89% for V 2O5-based cell. Also an increase in device efficiency from 8.78% to 9.65% was achieved using V2O5 as the new electron recombination blocking layer. The CV measurements revealed that TiO2/V2O5 based cell has higher capacitance than bare TiO2 which indicates that the former has higher performance regarding charge transfer from the to the conduction band of TiO2 and lower recombination rate to the electrolyte. The charge recombination resistance RCT at the TiO2/electrolyte interface was much higher in V2O5-based cells than that in conventional cells at various applied biases. Electron lifetime (τn) increased by a factor more than of ~3 in V2O5-based DSCs and this indicated that V2O5 significantly reduced the back charge transfer (recombination) from TiO2 to the electrolyte rate at TiO2/electrolyte interface.

Acknowledgement This study was partially supported by the US-Pakistan Joint Science and Technology, NSF/EPSCoR program (grant no. 0903804) and by the State of South Dakota, NASA EPSCoR program (grant no. NNX13AD31A), SDBoR CRGP, and Egyptian government fellowship (Egyptian Missions sector, 2012) for Mr Elbohy.

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13

Voltage (V)

0

0.0

(a)

0.4

0.6

0.8

C onventional DSSC V O based 2 5 cell

2

C urrent density (mA/cm)

-3

0.2

-6 -9 -12 -15 -18

1.0 2

(b)

Conventional DSSC V O based cell 2 5

Dark current density (mA/cm )

0.8

0.6

0.4

0.2

0.0 0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 1 (a) Illuminated and (b) dark current density – voltage (JV) curves for both conventional and V2O5 modified cells.

Table 1 Illuminated I-V parameters for typical and modified cell. Cell type V2O5 based cell Conventional cell

VOC (V) 0.83 ±0.002 0.8 ± 0.005

JSC (mA/cm2) 16.91 ± 0.07 15.64 ± 0.04

FF (%) 68.1 ± 0.1 70 ± 0.05

Efficiency (η) 9.56 ± 0.07 % 8.7 ± 0.1 %

14

Figure 2 Energy band diagram of DSCs (a) before and (b) after adding V2O5 showing its role to suppress back electron transfer (recombination) indicated by red arrows.

15

100 90 80

IP CE (%)

70 60 50 40 30 20

V2

Conventional DSC O 5 based cell

10 0 350 400 450 500 550 600 650 700 750 800

Wavelegnth (nm)

Figure 3 Incident photon-to-electron conversion efficiency (IPCE) action spectra for both the conventional and V2O5 modified cells.

16

Figure 4 Slow scan rate cyclic vooltammetry of (a) nanoporous titanium dioxide (b) TiO2/V2O5 composite in I-/I3- electrolyte. The he measurements were performed at 5 mV/sec sccan rate.

17

(a)

Figure 5. (a) Equivalent circuit, eelectrochemical impedance spectroscopy py of (b) conventional conven and (c) V2O5-based cells.

18

Table 2 Charge recombination resistance values for both conventional and V2O5 modified cells as a function of applied voltage. RCT (Ω.cm2) RCT (Ω.cm2) RCT( (Ω.cm2) RCT (Ω.cm2) Cell at 0.6 V at 0.65 V at 0.7 V at 0.75 V Conventional cell

30.47

17.6

9.23

3.67

V2O5-based cell

151.13

69.73

31.73

15.60

Table 3 Chemical capacitance values for both conventional and V2O5 modified cells as a function of applied voltage.

Cell

Cµ (F/cm2) at 0.6 V

Cµ (F/cm2) at 0.65 V

Cµ (F/cm2) at 0.7 V

Cµ (F/cm2) at 0.75 V

Conventional cell

3.22×10-4

4.49×10-4

5.13×10-4

5.27×10-4

V2O5-based cell

2.58×10-3

4.56×10-3

7.43×10-3

10×10-3

19

Figure 6 Comparison between ellectron lifetimes of both conventional and V2O5-based cells as function of applied voltage

20

1.0 DSSC+V O 2

0.8

5

Normalized Photovoltage

Conventional DSC 0.6

0.4 100 msec 0.2 30 msec 0.0 0.0

-2

2.0x10

-2

4.0x10

-2

6.0x10

-2

8.0x10

-1

1.0x10

Time (sec)

Figure 7 Transient photovoltage decays of both conventional and V2O5-based cells

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Developed solution processed vanadium pentoxide (V2O5) as a novel blocking layer in dye-sensitized solar cells (DSCs). Significantly increased cell efficiency from 8.78% to 9.65% and peak external quantum efficiency (EQE) from ~ 80% to ~ 88-89%. Exhibited longer charge recombination resistance (RCT) in V2O5 layer modified DSCs than conventional cells evidenced from electrochemical impedance spectroscopy and transient photovoltage measurements.

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A thin layer of solution processed V2O5 that suppresses the back electron transfer (recombination) in dye-sensitized solar cells. B

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iography - Hytham Elbohy Hytham Elbohy received his Bachelor’s Degree in Physics from Faculty Science, Mansoura University, Damietta Campus, Egypt in 2005. He also received his Master’s degree in Experimental Physics from Faculty of Science, Mansoura University, Damietta Campus, Egypt in 2010. He is currently pursuing his PhD degree in Electrical Engineering from South Dakota State University under the supervision of Dr. Qiquan Qiao. His research work includes nanostructured carbon, metal, metal oxides, and polymer films as electrodes for application in dye sensitized solar cells.

Biography - Prashant Poudel Prashant Poudel received his Bachelor’s Degree in Electrical Engineering from Institute of Engineering, Tribhuvan University, Kathmandu, Nepal in 2009. He obtained his Master’s degree in Electrical Engineering from South Dakota State University under the supervision of Dr. Qiquan Qiao in 2013. His research work includes nanostructured carbon, metal, metal oxides, and polymer films as electrodes for application in dye sensitized solar cells.

Biography - Qiquan Qiao Dr. Qiquan Qiao is an associate proffessor in the Department of Electrical Engineering and Computer Science at South Dakota State Univversity (SDSU), where he established the Organic Eleectronics Laboratory. Current research focuses ocuses on polymer photovoltaics and dye-sensitized solarr cell materials and devices. He received 2014 F O Buttler Award for Excellence in research, 2012 College of Engineering Young Investigator Award, 2010 US S NSF CAREER Award, and 2009 Bergmann Memor orial Award from the US-Israel Bi-national Science Founda Foundation (BSF). During his graduate study, Dr. Qiao ao received the 2006 American Society of Mechanical cal Engineers Solar Energy Division Graduate Student ent Award and the 2006 Chinese Government Award foor Outstanding Students Abroad.

Biography – Nirmal Adhikari Nirmal Adhikari received his Bachelor’s Degree in Electrical Engineering from Institute of Engineering, Tribhuvan University, Kathmandu, Nepal in 2006 and completed his Master degree in Materials and Process of Sustainable Energetics from Tallinn University of Technology, Eastern Europe in 2011. He is currently pursuing his Doctoral degree in Electrical Engineering from South Dakota State University under the supervision of Dr. Qiquan Qiao. His work includes solution processing of organic and inorganic materials and Physical Vapor Deposition (PVD) of inorganic semiconductor thin films for PV applications. His major focus is on the interface engineering of perovskite solar cell at nanoscale for efficient charge transport using Kelvin Probe Force microscopy (KPFM) and Transient Photoconductivity measurements.