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Nano Energy (2015) 11, 56–63

http://dx.doi.org/12211-2855/& 2014 P

nCorresponding auE-mail address: q

RAPID COMMUNICATION

Low temperature efficient interconnectinglayer for tandem polymer solar cells

Abu Farzan Mitul, Lal Mohammad, Swaminathan Venkatesan,Nirmal Adhikari, Sudhan Sigdel, Qi Wang, Ashish Dubey,Devendra Khatiwada, Qiquan Qiaon

Center for Advanced Photovoltaics, Department of Electrical Engineering, South Dakota State University,Brookings, SD, USA

Received 7 August 2014; received in revised form 28 August 2014; accepted 11 September 2014Available online 22 October 2014

KEYWORDSTandem structure;Interconnecting layer;Polymer solar cells;PEIE

0.1016/j.nanoen.2ublished by Elsevi

thor.iquan.qiao@sdsta

AbstractTandem solar cells are found to be promising in organic photovoltaic research because they candouble open-circuit voltage (Voc) and utilize photon energy efficiently, as compared to singlejunction solar cells. In a typical tandem polymer solar cell, two single subcells are staked andconnected by an interconnecting layer, which is a prerequisite to achieving high photo-electronconversion efficiency. In this work, a low temperature solution processed and chemically stablePEDOT:PSS/AZO/PEIE interconnecting layer is reported. Tandem polymer solar cells based onthis efficient layer can realize a high Voc, which can sum the Voc's of subcells. Furthermore, theoperation mechanism of the interconnecting layer is discussed to help better understand theelectron recombination process in the interconnecting layer.& 2014 Published by Elsevier Ltd.

Introduction

Organic solar cells have attracted much attention in thepast decade, because of their solution based processing andpotentially low cost as renewable energy resources. How-ever, one problem to common organic solar cells is that theabsorption spectral range of donor polymers is narrow,which limits the solar energy utilization of the entire

014.09.026er Ltd.

te.edu (Q. Qiao).

spectrum. To achieve a broadband absorption, tandemstructures have been employed by stacking two or morebulk heterojunction (BHJs) with complementary absorptionspectra. The open circuit voltage (Voc) of a tandem cell istypically the sum of Voc's of individual subcells, while theoverall current density is limited by the subcell with lowercurrent density. Tremendous efforts have been devoted tothe development of desirable device configurations andinterconnecting layers (ICLs) between subcells. Tandemcells with a power conversion efficiency (PCE) over 10.6%have been achieved [1]. To construct high efficiency tandempolymer solar cells, high-performance individual subcells

57Low temperature efficient interconnecting layer for tandem polymer solar cells

using polymers with complementary absorption are needed[2]. Additionally, a tandem solar cell requires an ICLbetween subcells [3]. Ideally, the ICL and subcells shouldform ohmic contact, and the ICL also should function as acharge recombination layer to complete the circuit so thatthe Voc can add up. In inorganic tandem solar cells, theinterconnecting layer is a tunneling diode consisting ofheavily doped n- and p-type layers. In their organic counter-parts, small moleculare tandem solar cells are simplerbecause the interconnecting layer and the top subcell arethermally evaporated. For example, Leo et al. evaporatedhighly p-doped N,N'-diphenyl-N,N'-bis(4-(N,N-bis(naphth-1-yl)-amino)-biphenyl-4-yl)-benzidine (DiNPB) and n-doped C60directly connecting two cells as a recombination layer [4].The evaporation based approach has disadvantages includingcomplicated process and high cost. Solution processed poly-mer tandem cells also have challenges. In addition to opticaltransparency and electrical functionality, the interconnec-tion layer coating should not damage the existing subcells.Thus, the solvent selection is critical [2]. The penetration ofsolvents through the ICL may disturb charge carrier selectiv-ity of the ICL between two subcells, and thus enhance thecharge recombination loss [3]. Therefore, a reduced Voc iscommonly observed for tandem cells. Furthermore, oncecoated, the ICL should provide protection so that thesequential BHJ active layer of the second subcell will notdamage the underlying layers. Therefore, the ICL should havemechanical robustness in addition to device functionality.

The knowledge regarding interfacial layers accumulated inthe organic light emitting diode (OLED) and organic photo-voltaic (OPV) research fields provided strong support fortandem OPV interconnecting layer development. In 2006,Kawano et al. demonstrated a polymer tandem solar cellusing sputtered indium tin oxide (ITO) combined with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)as inter-connecting layer [5]. However, there was a loss in Vocof the tandem cell compared to the reference single cellsbecause of the high contact resistance between the ITO andbottom cell interface. Janssen et al. showed an efficient ICLconsisting of n-type and p-type layers, where the n-type layeris a bi-layer structure lithium fluoride/aluminum (LiF/Al) andthe p-type layer is a bi-layer of gold (Au)/PEDOT [6]. However,the metal (Au) layer in this ICL is thick, which leads to a loss intransmittance of photons for the rear subcell. In addition,both the p- and n-type layers are formed through vacuumdeposition, not all solution processed. Solution-processedelectron transport layers such as zinc oxide (ZnO) and titaniumdioxide (TiO2) have been used as n-type interlayer in singlejunction polymer solar cells [7–13]. These electron-transporting layers are wide band gap semiconductors andare highly transparent in the visible and near-infrared (NIR)wavelength range where the polymer solar cells absorb. Onthe p-type side, PEDOT:PSS is an excellent hole-transportinglayer for polymer solar cells [14]. It has been reported that theregular PEDOT:PSS layer is not strong enough to provideprotection. The solution to this problem is using a low boilingtemperature solvent (e.g., chloroform) for the deposition ofthe second subcell. High performance polymer solar cells aretypically based on 1,2-Dichlorobenzene (ODCB) or chloroben-zene (CB) because they can facilitate morphology deve-lopment for effective charge transport and collection.To overcome the challenge of making high performance

tandem solar cells using higher boiling point solvent, Yanget al. introduced surfactant and dimethylformamide (DMF)into PEDOT:PSS to improve the mechanical properties and theconductivity of PEDOT:PSS. The modified PEDOT:PSS layerbecame very robust and showed excellent electrical conduc-tivity and transparency [3,15]. PCE of 5.84% with a regulartandem device architecture utilizing an Al-TiO2–PEDOT:PSSinterlayer was achieved by Srista et al. in 2010 [1]. Aninverted tandem cell with a PCE of 8.62% using modifiedPEDOT:PSS-ZnO interlayer was achieved by Dou et al. in 2012.[15] There were issues with ZnO stability such as aggregationof ZnO nanoparticles. Repeated ultraviolet (UV) illumination isrequired to remove the “S shape” current density–voltage (J-V) curves in ZnO/PEDOT:PSS interlayer and form ohmic contactat the interconnecting layer [16]. Room temperature proces-sable metal oxides are ideal materials for developing efficientand stable single layer and multilayer organic solar cells,though several commonly employed metal-oxide interlayerssuch as ZnO demanding post-thermal annealing to attainimproved conductivity [17]. Such high temperature processingis a drawback for realizing competitive organic solar cells.Generally speaking, an ideal interconnecting layer needs topossess an energy level matching with those of donor andacceptor molecules in the active layer, sufficient conductivity,high transparency, uniform coverage and good chemicalstability. Several interconnecting layers including TiOx/PEDOT:PSS [15], ZnO/PEDOT:PSS [3], molybdenum trioxide(MoO3)/Al/ZnO [18], Al/TiO2/PEDOT:PSS [1], LiF/Al/Au/PEDOT: PSS [6], LiF/Al/MoO3 [19], and MoO3/Ag/Al/Calcium(Ca) [20] have recently been demonstrated. More recently,graphene oxide (GO) based interconnecting layers, such asZnO/GO:PEDOT:PSS [21] and ZnO/GO:single-walled carbonnanotube (SWNTs) [22] have also been reported. Because ofthe low conductivity of GO, however, certain highly conduc-tive materials (e.g., PEDOT:PSS, SWNTs) have to be blendedinto the GO interconnecting layer, which complicated thedevice fabrication and limited the Voc of tandem cells to beonly �80% of the sum of that of the subcells. There is a needto provide stable and optical transparent interconnectinglayers that can be processed by low temperature solution [23].

Brabec et al. demonstrated aluminum-doped zinc oxide(AZO) layers from a sol-gel precursor, which can be pro-cessed at lower processing temperatures (o150 1C). AZOwith lower processing temperatures (o150 1C) offers asuitable temperature for plastic substrates and depositiononto organic films [24] because some medium and lowbandgap polymers cannot bear high temperatures. Lowtemperature solution processed interconnecting layer is arequirement for stacking two or more bulk heterojunction(BHJs) in tandem solar cells to achieve a broad absorption.Previous research revealed that the S-shape curves could bederived from an interfacial barrier [25]. Ethoxylated poly-ethylenimine (PEIE) can facilitate electron transport bylowering the energy barrier between the active layer andITO cathode in BHJ devices [26]. Here, we report PEDOT:PSS/AZO/PEIE as a low temperature processable, efficientinterconnecting layer for tandem polymer solar cells.Devices based on this efficient layer can realize high Voc's,which can sum the Voc's of subcells. The operation mechan-ism of the interconnecting layer is also discussed to helpunderstand the electron recombination process in theinterconnecting layer.

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Experimental procedure

Materials

Regioregular P3HT (rrP3HT) was purchased from RiekeMetals. PC60BM was ordered from Nano-C. MoO3, PEIE, ZincAcetate, Aluminum Nitrate, nonahydrate Ethanol and Nitro-benzene were purchased from Sigma Aldrich. PEDOT:PSSand silver were purchased from Heraues Materials Technol-ogy LLC and Kurt J Lesker Company respectively. Allmaterials were used without further purification. AZO sol-gel was synthesized using the procedures described byStubhan et al. [24]. Here, 2.17g of zinc acetate dehydrate[Zn(CH3COO)2 � 2H2O] and 3.8 mg of aluminum nitrate non-ahydrate [Al(NO3)3 � 9H2O] were dissolved in 100 ml ethanolat 80 °C for 2.5 hours. Finally, a 0.45 µm PVDF filter wasused to remove any precipitates from the AZOsol-gel. A total of 0.1 wt% of PEIE was mixed with2-methoxyethanol (ME) in the interconnecting layer.

Single-junction device fabrication

ITO coated glass slides were cleaned by ultra-sonication for20 minutes in detergent water, de-ionized water, acetoneand 2-propanol sequentially. The ITO substrates were thensubjected to oxygen plasma cleaning for 25 minutes beforespin-coating AZO layer. AZO was coated at 2000 rpm for1 minute and then kept on hot plate at 150 1C for 10 minutesto remove residual solvent. AZO coated substrates were thentransferred to nitrogen filled glove box having lower than1 ppm of O2 and H2O. Nitrobenzene was added to the blendsolution with concentration of 4 vol% 30 minutes prior to spincoating. The blend solution P3HT:PCBM (1:0.7) was spin-coated at 1000 rpm for 45 seconds and annealed at 150 1C for

Figure 1 (a) Tandem device structure using 3 different inter-connecting layers and (b) energy level diagram of tandempolymer solar cell.

10 minutes. Then, the sample was transferred to a thermalevaporator where 10 nm of MoO3 and 80 nm of silver (Ag)were thermally evaporated in vacuum below 2� 10�6 Torr.

Tandem device fabrication

The fabrication of the front subcell followed the procedure forthe preparation of the single-junction device until the activelayer was deposited. A conductive PEDOT:PSS (80 vol% CleviosCPP105D, 20 vol% IPA) was spin-coated on top of bottom activelayer at 4500 rpm for 2 minutes. AZO was spin coated on top ofPEDOT:PSS layer at 3000 rpm for 1 minute and annealed at150 1C for 5 minutes. On top of AZO layer, PEIE layer wascoated at 5000 rpm and annealed at 120 1C for 2 minutes. Theback subcell was fabricated in the same manner as the frontsubcell, with the rate of spin coating ranging from 1200 to1400 rpm. Finally, MoO3 and Ag were thermally evaporated asdescribed for the single junction cell.

Device characterization

Agilent 4155C semiconductor parameter analyzer was usedfor J-V characteristic measurements. The semiconductor

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Figure 2 UV–vis absorption spectra of P3HT:PCBM at 1000 rpmand 1400 rpm, respectively.

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Figure 3 Transmission spectra of PEDOT:PSS/AZO/PEIEinterconnecting layer.

Figure 4 AFM images of (a) Bottom active layer P3HT:PCBM, (b) PEDOT:PSS on top of active layer, (c)PEIE on top of PEDOT:PSS, (d)AZO on top of PEDOT:PSS, (e) PEIE deposited on top of AZO and PEDOT:PSS, (f) Top cell (P3HT:PCBM) on top of ITO/Bottom.

59Low temperature efficient interconnecting layer for tandem polymer solar cells

parameter analyzer is integrated with a Newport xenon lampas a solar simulator (AM 1.5). The light intensity of light sourceis 100 mWcm�2. A National Renewable Energy Laboratory(NREL) calibrated silicon photodetector was used to calibratethe light source for J-V characteristic measurements.

Agilent 8453 spectrophotometer was used to perform UV-Vis absorption. Atomic force microscopy (AFM) images were

obtained in a tapping mode with Agilent 5500SPM. We usedBudget Sensors (TAP 300EG) Cr/Pt coated silicon tips with aresonance frequency of �300 KHz. The AFM images wereanalyzed by Gwyddion software.

Rigaku SmartLab diffractometer was used to record X-raydiffraction (XRD) patterns where the patterns wererecorded utilizing parallel beam configuration. A computer

A.F. Mitul et al.60

controlled Ametek VERSASTAT3-200 potentiostat was usedfor electrochemical impedance spectroscopy (EIS). Therewas a frequency analysis module (FDA) along with thepotentiostat to obtain cole-cole plot of the devices. TheAC signal of amplitude 10 mV was maintained and thefrequency range was from 1 to 106 Hz under dark conditions.Data extracted from electrical impedance spectrum mea-surements was used to obtain capacitance vs. frequencyplots using VersaStudio Software.

Results and analysis

Figure 1a and b show device structure and energy leveldiagram of polymer tandem solar cell using PEDOT: PSS/AZO/PEIE interlayer. P3HT:PCBM blend was used as activelayer on both of the subcells of the tandem device.Polymers with complementary absorption capabilities canutilize solar spectra in a more efficient way. However, thepurpose of this research is to study the interconnectinglayer. Thus we used P3HT in both subcells. Figure 2 showsUV–vis absorption spectra of P3HT:PCBM which was used asactive layer in both subcells of tandem device. Bottom cellwas coated at higher rpm (1400 rpm) in order to obtaincurrent matching in both subcells. More than 80% of incidentlight reaching the photoactive layer is good for sufficientlight absorption. Figure 3 shows that PEDOT:PSS/AZO/PEIEhas average transmittance greater than 85% with a peaktransmittance of �85% in the visible spectral range.

It is possible that the improvement in the device perfor-mance is due to better surface quality of the film, which

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Figure 5 Current density-voltage characteristic curves ofsingle junction and tandem solar cell using 3 different inter-connecting layers.

Table 1 Photovoltaic parameters of single and tandem solar c

ICL Voc (V) (Avg) Jsc (mA/cm

Single Junction 0.6 (0.593) 8.68 (8.42PEDOT:PSS/AZO 1.1 (0.98) 5.23 (4.96PEDOT:PSS/PEIE 0.8 1.49PEDOT/AZO/PEIE 1.13 (1.09) 5.34 (5.27

provides better wettability and adhesion with photoactivelayer [27]. The topography and surface roughness of thefilms were investigated by atomic force microscopy (AFM) inFigure 4. Figure 4a shows surface of P3HT:PCBM (bottomcell) on top of AZO. Topography image of P3HT: PCBM showsfully covered rough surface where the peaks are of 6.17 nm.However, there is no significant grain boundary observed.Figure 4b shows the coated layer of PEDOT:PSS on top ofP3HT:PCBM. surface roughness/root mean square (rms)value decreased to 6.1 nm. After coating PEIE on top ofPEDOT:PSS (Figure 4c), the surface seems to be porous andthere exists small holes on surface. This porous surfacestructure might affect the charge transport in device. Theporous surface morphology can be explained as partialdissolution of PEDOT:PSS layer due to the top PEIE layer.When AZO is coated on top of PEDOT:PSS (in Figure 4d),there appears small grains of size 20 nm on the surface.Small grain size reduces the surface roughness. The surfaceroughness of AZO deposited on top of PEDOT:PSS is about2.62 nm. Larger grain size increases interfacial area andthus increases conductivity. Larger grain appears inFigure 4d and e which provide broad hill-like features onsurface of the layers. PEIE layer makes the surface of themetal oxide layer smooth leading to improved wettabilityand adhesion between the active layer and the metal oxide.It is obvious that the PEIE layer is an effective interfaciallayer between the active layer and the electrode to improvedevice efficiency [26]. The rms of the films is 5.83 nm and5.11 nm respectively. Increased contact area between thepolymer film and the metal anode for the films with higherrms may result in a more efficient charge collection at theinterface [32]. Again, Figure 4a and f show the morphologyof P3HT:PCBM both as bottom and top subcell respectively.There is an obvious difference between the morphology ofsame active layer (P3HT:PCBM). In top active layer (P3HT:PCBM), polymer agglomerates and interfaces form while inbottom active layer there is no significant grain boundaries.So, topography images show that PEDOT:PSS/AZO/PEIE canprovide favorable surface morphology for efficient chargetransport and collection at the interfaces between thepolymer film and the metal anode.

The J-V characteristics of the single and tandem cells andtheir performance parameters are shown in Figure 5. Table 1shows single junction and tandem solar performances forthree different interconnecting layers. PEDOT:PSS/AZO/PEIEinterconnecting layer showed a PCE of 3.58% where theaverage was 3.28%. There was significant improvement indevice performance compared to the other two intercon-necting layers. PEDOT:PSS/AZO showed a highest Voc of 1.1Vwhere �0.98 V was the average. Average Jsc and FFwere around 4.96 mA/cm2 and 48.3% for PEDOT:PSS/AZO

ells.

2) (Avg) FFmax (%) (Avg) PCE(%) (Avg)

) 63.7 (62.9) 3.26 (3.14)) 55.6 (48.3) 3.2 (2.33)

13 0.15) 59.3 (56.9) 3.58 (3.28)

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Figure 7 Impedance spectrum of tandem devices with differ-ent interconnecting layer in the dark.

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Figure 8 Capacitance vs. frequency plots of tandem solar cellfor different interconnecting layer.

61Low temperature efficient interconnecting layer for tandem polymer solar cells

interconnecting layer. Using interfacial dipole between elec-tron transport layer and active layer, device performanceimproved. In tandem solar cell, interfacial dipole canimprove the degree of electron extraction from the frontsubcell, lower the energy barrier for electron injection andminimize charge trapping at interface [28]. Kyaw et al.reported that PEIE layer formed interfacial dipoles at ZnO/polymer interfaces in single junction device, leading to areduced work function (WF) of ZnO from 4.5 eV to 3.8 eV[26,29]. Again, the formation of an interfacial dipole layercan be confirmed by small-angle XRD pattern [30]. Figure 6shows small angle XRD pattern of PEIE layer on top of cleanedITO substrates. The XRD feature shows a strong peak ataround 0.7 degree two theta angle which indicates theexistence of ordered structure of PEIE layer. Here, we assumethat PEIE dipoles are continuously aligned with each otherproviding a crystalline structure in the film. So, introducingPEIE on top of AZO improved FF, Jsc and Voc of the tandemdevice. PEDOT:PSS/AZO/PEIE as an interconnecting layershowed the highest Jsc, Voc and FF around 5.34 mA/cm2,1.13 V and 59.3% respectively, where the average valueswere 5.27 mA/cm2, 1.09V and 56.9% respectively. Consistentresults reveal that PEDOT:PSS/AZO/PEIE interconnectinglayer can connect top and bottom cell successfully. Compar-ing to PEDOT:PSS/AZO/PEIE and PEDOT:PSS/AZO, PEDOT:PSS/PEIE showed poor device performance. From the AFM images(see Figure 4), it was clear that PEDOT:PSS/PEIE cannot beused as an efficient interconnecting layer. Single junctionsolar cell provided Voc of about 0.6 V which shows that thereis some recombination loss in PEDOT:PSS/AZO/PEIE interfa-cial layer for tandem cells. This loss might be due to theincomplete recombination at the interfacial layer.

To further investigate the photovoltaic parameters oftandem devices with different interconnecting layer combi-nations, we performed the electrical impedance spectrummeasurements which enabled us to monitor the specificelectrical properties of the interfaces [23,31–34]. Figure 7shows the Cole-Cole plot (the imaginary resistance, Zimover the real resistance, Zre) of tandem devices withdifferent interconnecting layers in the dark. The arc forPEDOT:PSS/PEIE is larger than that for devices of PEDOT:PSS/AZO and PEDOT:PSS/AZO/PEIE interlayer. The resis-tance of the interconnecting layer decreased in the orderof PEDOT:PSS/PEIE, PEDOT:PSS/AZO and PEDOT:PSS/AZO/

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Figure 6 Small angle X-ray diffraction pattern of PEIE layer ontop of ITO substrate.

PEIE. Interfacial dipole PEIE layer enhances the conductivityas well as increases charge extraction and recombinationprocess in metal oxide interfacial layer (PEDOT:PSS/AZO/PEIE). The frequency dependence of the capacitance of thesolar cell is shown in Figure 8 where the low frequencyplateau shows a higher capacitance in the order of PEDOT:PSS/AZO/PEIE, PEDOT:PSS/AZO and PEDOT:PSS/PEIE. Porousinterfacial layer increases capacitance of the device due totrapped charges (see Figure 4c). However, this loss can beminimized either through optimized layer thickness orincreasing the conductivity at interfacial layer.

Conclusion

In this work, we have designed and fabricated tandem solarcells using a low temperature solution processed interconnect-ing layer, PEDOT:PSS/AZO/PEIE. Nano morphological changes ininterfacial layer have found to show great impact on tandemdevice performance. It was demonstrated that surface modifierPEIE can contribute to favorable morphology in interconnectinglayer for efficient charge transport and recombination.

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Acknowledgments

This research was benefited from the grants including NSFCAREER (ECCS-0950731), NASA EPSCoR (NNX13AD31A) andNSF MRI (grant no. 1229577).

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Abu Farzan Mitul received his Bachelor`sdegree in Electrical and Electronic Engineer-ing from the Khulna University of Engineeringand Technology (KUET), Bangladesh in 2011.He is currently pursuing his Master’s degree inElectrical Engineering from the South DakotaState University under the supervision of Dr.Qiquan Qiao. The subject of his thesis is toinvestigate the optimized interfacial condi-tion for double and triple junction polymer

solar cells. His research interest includes Hybrid Tandem Solar Cells,Singlet fission based solar cell, Solvent-Free Bulk Heterojunction (SF-BHJ) organic photovoltaics, interfacial engineering in organic photo-voltaics and NanoPhotonics.

Lal Mohammad received his Bachelor’sDegree in Electrical and Electronic Engineer-ing from the Bangladesh University of Engi-neering and Technology (BUET). He iscurrently pursuing his Master’s degree inElectrical Engineering from the South DakotaState University under the supervision of Dr.Qiquan Qiao. His research work includes studyof charge transport and efficient intercon-necting layers for tandem polymer solar cells.

Swaminathan Venkatesan received his Bache-lor’s degree in Metallurgical and MaterialsEngineering from the National Institute ofTechnology, Trichy, India in 2006. He thencompleted his Masters in Materials Scienceand Engineering from the University of South-ern California, Los Angeles, USA. He recentlydefended his Doctoral dissertation under theguidance of Dr. Qiquan Qiao titled “Engineer-ing nanomorphology in polymer solar cells for

efficient charge transport”. He is currently working as a ResearchAssociate under the guidance of Dr. Cheng Zhang where his researchwork includes Morphology and Charge Transport characterization ofSterically Protected Chromophores and Polymers.

63Low temperature efficient interconnecting layer for tandem polymer solar cells

Nirmal Adhikari received his Bachelor’sDegree in Electrical Engineering from theInstitute of Engineering, Tribhuvan Univer-sity, Kathmandu, Nepal in 2006 and com-pleted his Master degree in Materials andProcess of Sustainable Energetics from theTallinn University of Technology, EasternEurope in 2011. He is currently pursuinghis Doctoral degree in Electrical Engineeringfrom South Dakota State University underthe supervision of Dr. Qiquan Qiao. His

major research is on the interface engineering of perovskite solarcell at nanoscale for efficient charge transport using Kelvin ProbeForce microscopy (KPFM) and Transient Photoconductivitymeasurements.

Sudhan Sigdel received his Bachelor’sDegree in Electronics and CommunicationEngineering from the Institute of Engineer-ing, Tribhuvan University, Kathmandu,Nepal in 2011. He is currently pursuing hisMaster’s degree in Electrical Engineeringfrom the South Dakota State Universityunder the supervision of Dr. Qiquan Qiao.His research work includes nanostructuredcarbon, metal oxides, and polymer films as

electrodes for application in dye sensitized solar cells.

Dr. Qi Wang received his Dr. Degree fromthe Changchun Institute of Applied Chemis-try, CAS in 2009. Then he spent 3 years atUniversity of Texas at Dallas and Universityof North Texas doing postdoc research. Bythe end of 2012, he was appointed a fullprofessor by Faculty of Science, Xi’an Jiao-tong University, China. In January 2014, hejoined Dr. Qiquan Qiao group as a visitingscholar. He has published over 30 peer-

reviewed papers which have received over 1600 times citationsfrom 2008. His research interests include triplet materials for OSCsand OLEDs, device physics of organic semiconductor devices.

Ashish Dubey is a graduate student inElectrical Engineering at the South DakotaState University, USA. He received his mas-ter degree in Nanotechnology from AmityUniversity, India. Currently he is carryingout his doctoral thesis work in Dr. QiquanQiao group. His research work includes,perovskite based solar cells, organic–inor-ganic semiconductor hybrid solar cells,structural and morphological studies of

donor–acceptor blend films, and their optical and electricalcharacterization.

Devendra Khatiwada received his Master’sdegree in Physics from the Prithvi NarayanCampus, Tribhuvan University, Nepal in2009. He is currently pursuing his Master’sdegree in Electrical Engineering from SouthDakota State University under the super-vision of Dr. Qiquan Qiao. His research workincludes morphology study of PolymerBulkheterojunction solar cell.

Dr. Qiquan Qiao is an associate professor inthe Department of Electrical Engineeringand Computer Science at the South DakotaState University (SDSU), where he estab-lished the Organic Electronics Laboratory.His current research focus is on polymerphotovoltaics and dye-sensitized solar cellmaterials and devices. He received 2014F.O. Butler Award for Excellence inresearch, 2012 College of Engineering Young

Investigator Award, 2010 US NSF CAREER

Award, and 2009 Bergmann Memorial Award from the US–Israel Bi-national Science Foundation (BSF). During his graduate study, Dr.Qiao received the 2006 American Society of Mechanical EngineersSolar Energy Division Graduate Student Award and the 2006 ChineseGovernment Award for Outstanding Students Abroad.