Utility of copper oxide nanoparticles (CuO-NPs) as efficient electron donor material in bulk-heterojunction solar cells with enhanced power conversion efficiency

The present piece of work successfully fabricates P3HT:PC 70 BM solar cells by incorporating wet chemically synthesized CuO nanoparticles to adjust the morphology of the active layer by [r]

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Original Article

Utility of copper oxide nanoparticles (CuO-NPs) as efﬁcient electron

donor material in bulk-heterojunction solar cells with enhanced

power conversion efﬁciency

Hafsa Siddiquia,b,*,1, Mohammad Ramzan Parraa,c,1, Padmini Pandeya,d, M.S Qureshia, Fozia Zia Haquea,**

aOptical Nanomaterial Lab, Department of Physics, Maulana Azad National Institute of Technology, Bhopal, 462003, India bDepartment of Physics, Sha-Shib College of Science and Management, Bhopal, 462030, India

cDepartment of Physics, Govt Degree College Boys Sopore, Jammu&Kashmir, 193201, India dDepartment of Physics, Savitribai Phule Pune University, Pune, 411007, India

a r t i c l e i n f o

Article history:

Received October 2019 Received in revised form 18 January 2020 Accepted 23 January 2020 Available online xxx

Keywords:

Bulk heterojunction Solar cells

Copper oxide nanoparticles Thinﬁlms

Photo current density External quantum efﬁciency

a b s t r a c t

In the present work, we have endeavored the utilization of wet-chemically synthesized copper oxide nanoparticles (CuO-NPs) as the active layer in hybrid bulk heterojunction (BHJ) solar cells The BHJs with CuO-NPs display significantly different physics from customary BHJs, and prove a noteworthy improvement in their performance It is noted that with the addition of CuO-NPs, the morphology of the photoactive layer endures significant changes Incorporating CuO-NPs is an additional paradigm for BHJs solar cells which enhances the photocurrent density from 9.43 mA/cm2to 11.32 mA/cm2and the external quantum efficiency as well Also the power-conversion efficiency (PCE) improved from 2.85% to 3.82% without harming the open circuit voltage and thefill factor The enhancement in PCE achieved here makes it worthy to design high-performance organic solar cells holding inorganic nanoparticles ©2020 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Currently, in order to adapt to the rapid development of elec-tronic devices and electric vehicles, various energy storage mate-rials are constantly being designed and developed Bulk heterojunction solar cells (BHJ-SCs) have many advantages such as low cost of fabrication and an easy and simple fabrication process with a wide range of applications They have many tremendous features such as transparency and the possibility of being fabricated in different colors, thus being of interest for building-integrated

photovoltaics (BIPV) applications [1,2] BHJs comprise of several layers in which the photoactive layer plays a crucial role in enhancing the overall photo-conversion efficiency (PCE orh) The main challenging fact that is highlighted in the literature for BHJ-SCs is the poor light absorption mainly due to the small exciton diffusion length and short carrier mobility [3] To cover the visible region of the solar spectrum, it requires compounds that strongly absorb this range [4] Therefore, a combination of inorganic nano-particles with P3HT:PCBM (poly(3-hexylthiophene): phenyl-c61-butyric acid methyl ester), have a potential to surpass in better performance while retaining the benefits Inorganic nanoparticles have features as bandgap tunability, high absorption coefficient and high intrinsic charge carrier mobility [5,6] Moreover, previous studies of solar cells that have directly incorporated inorganic nanoparticles as electron acceptors i.e., ZnO, TiO2, or FeS2 nano-particles, consist of light-harvesting absorbers, or light-scattering centers using Au, Ag or PbS nanoparticles in conjugated polymer films [7e9] Compared to these inorganic nanoparticles, CuO nanoparticles, a photo-generating material, have higher absorption in the visible region and inject excess electrons to the structure *Corresponding author Department of Physics, Sha-Shib College of Science and

Management, Bhopal, 462030, India

**Corresponding author Optical Nanomaterial Lab, Department of Physics, Maulana Azad National Institute of Technology, Bhopal, 462003, India

E-mail addresses: hafsa.phy02@gmail.com (H Siddiqui), foziazia@rediffmail com(F.Z Haque)

Peer review under responsibility of Vietnam National University, Hanoi Equal contribution: Hafsa Siddiqui and Mohammad Ramzan Parra made an equal contribution

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2020.01.004

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[10e12] Much research has been carried out in theﬁeld of catalyst, sensor and energy conversion due to the contribution of CuO [13e18] The wide applications of CuO with controllable size, shape, defect and dopant has intensely inspired many researchers The wide-range studies carried out show that the development of cupric oxide (CuO) nanocrystals with modiﬁed architectures es-tablishes a relationship between the structure and the properties of CuO and its practical applications [19e22] Hence, the P3HT donor property could be tuned by generating electrons from the CuO nanoparticles M Ikram et al [23,24], E Salim et al [25] and A P Wanninayake et al [26], used commercially available CuO nano-particles to enhance the PCE of P3HT:PCBM solar cells

Here, we have synthesized CuO-NPs (for experimental details see electronic supporting information) by utilizing the wet chem-ical method and explained there structural, chemchem-ical and optchem-ical properties and followed the photovoltaic performance by serving them in P3HT:PC70BM in different concentrations (0%, 1%, 3%, 5%, 7%, and 10 wt %) Without the addition of CuO-NPs a PCE of 2.84% has been achieved for P3HT:PC70BM solar cells However, a higher efﬁciency of 3.82% is effectively achieved for CuO added P3HT:PC70BM because of an efﬁcient excitation generation, better light absorption and a photoexcited charge separation and collec-tion The concept of the CuO-NPs fabrication and the use of them into a P3HT:PC70BM photoactive blend is a noteworthy contribu-tion The systematic study with detailed discussion in the present

work is aﬁrst contribution towards the full understanding of such a device architecture

2 Experiments

All the experimental details are reported in Electronic Sup-porting Information (ESI)

3 Results and discussion

The XRD pattern of the prepared CuO nanoparticles (Fig 1a) confirms the formation of the pure monoclinic phase of CuO as all the marked peaks are well indexed with JCPDS card no 80-0076 In addition, the complete crystallographic information, as revealed through a Rietveld refinement of the prepared sample, is given in the supporting Information The refinement pattern is illustrated in Fig 1b

The micro Raman (m-RS) study further supports the micro-structural (crystallographic) changes and various defect states present in the prepared sample (Fig 1c) The peak found at 288 cm1is assigned to the Agmode, which corresponds to the typical motion of the oxygen atom for displacement in the b-di-rection of the monoclinic structure of CuO (for details please see [27]) Additionally, two peaks observed at 338 cm1and 624 cm1 are attributed to theﬁrst-order Raman (Bg) modes

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Further, the XPS survey scan does not include any chemicals other than Cu, O, and C as shown inFig 1d In addition (seeFig 1e), the core level scan spectrum of Cu2p shows a doublet with peaks centered at ~934.9±0.1 eV and ~954.3±0.1 eV corresponding to Cu2p3/2 and Cu2p1/2, respectively These peaks are accompanied with a set of satellites peaks at 962.2 eV, 941.3 eV and 943.6 eV corresponding to Cu2ỵstate in CuO [28] A spectral deconvolution of the O-1s spectrum (Fig 1f), results in two components appearing at around 531.02 eV and 532.36 eV The binding energy component observed at 531.02 eV corresponds to the O2 ion in the CueO bonds The peak observed at higher binding energy at around 532.36 eV relates to oxygen vacancies in the CuO lattice

Moreover, morphological investigations were performed using TEM with low and high magniﬁcations (Fig 1g and f) TEM images of the sample show size, shape and distribution of CuO-NPs as uniform and homogeneous The spherical nanoparticles have a diameter of ca 50±2 nm (see insetFig 1g) A selected-area of the electron diffraction pattern of CuO-NPs is indexed using C-Spot software The TEM diffraction pattern designates the presence of a single crystal with a monoclinic structure (see Fig 1i) The TEM results are well in accordance with the XRD results

Moreover, the optical band gap as well as the absorbance of the as-prepared CuO-NPs is a key factor that has a major effect on the performance of the prepared BHJs The obtained absorption spec-trum at ~836 nm corresponds to an energy of 1.47 eV (using tauc relation detail is given in electronic supporting information andFig S1) and is blue shifted to the visible region as compared to the reported absorption of CuO-NPs with an average particle size of ~50 nm (commercially available CuO-NPs) [23e26] Therefore, a better absorption of visible light is evidence of a better light harvesting

The above data conﬁrm the pure phase formation of the prepared CuO-NPs (detailed discussion above) These CuO-NPs were utilized

as a photo-absorber in the poly (3-hexyle thiophene) (P3HT) [6]: phenyl-C61-butyric-acid-methyl-ester (PCBM) solar cell device application We were able to achieve a remarkable enhancement in efficiency after inclusion of CuO-NPs The performance of the as-prepared CuO-NPs combined P3HT:PC70BM films were initially examined in detail via AFM, XRD and UV-visible spectroscopy The relevantfilms were spin cast on quartz substrates [29] The nano-scale morphology of pristine P3HT:PC70BM (Fig 2a) and CuO incorporated P3HT:PC70BMfilms (Fig 2bec) confirm the surface peaks of the CuO incorporated P3HT:CuO: PC70BM which are higher as compared to pristine P3HT:PC70BM and infer an obvious increase in surface roughness due to the addition of CuO-NPs The root-mean-square roughness (RMS) value increased from 0.711 nm to 4.188 nm as the addition of CuO-NPs increased from to 10 wt% The cell containing wt% of CuO-NPs shows a surface roughness value of 2.402 nm, because of an increased nanoscaled phase separation concerning the crystalline P3HT and the PC70BM acceptor [30,31] However, the surface roughness of thefilm which contain 10 wt.% of CuO may also increase the structural defects such as micro-cracks (seeFig 2c) which act as active recombination centers lead to in-crease the series resistance and lowering the Jsc an Vocvalues Optimal surface roughness gives more room for P3HT to form, thereby increasing crystallinity Furthermore, it can increase the interfacial contact area between the PEDOT:PSS and P3HT:CuO:PC70BM layer, allowing an efficient gathering of holes at the anode and thereby improving current density (Jsc) The incor-poration of CuO to the P3HT:PC70BM also affects the P3HT crystal-linity as supported by the XRD results (Fig 3a) The addition of copper nanoparticles can improve the crystallinity of P3HT [24] The observed increase in crystallinity of the P3HT state seems to be partially accountable for the rise in the absorbance and PCE of the devices [23] The Uv-visible absorbance spectra of pristine P3HT:PC70BM and CuO incorporated P3HT:PC70BM (Fig 3b) show

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two absorption zones Theﬁrst zone below 350 nm was recognized as PC70BM molecules while the absorption spectra from 350 nm to 650 nm (second zone) are related with poly (3-hexylthiophene) (P3HT) The peak obtained at ~500 nm can attributed to the pep* transition The region below the absorption peak shows the light harvesting ability of the photoactive layer [30] The obtained peak has exhibited a red shift ~510 nm after the incorporation of CuO-NPs, because of the interruption of the structure and the orientation of chain ordering of P3HT due to the CuO-NPs ability of light capturing In CuO incorporated photoactive layer blend, the absorption area is enhanced from visible light to the near infrared area The absorption is enhanced by the increasing amount of CuO nanoparticles in the active layer (InsetFig 3b)

Further, the performance of the as-prepared CuO nanoparticles in P3HT:PC70BM solar cell was examined The complete procedure of device fabrication and testing as well as cell parameters is pro-vided in the supporting information Thefill factor (FF), short circuit current density (Jsc), open circuit voltage (Voc), power conversion efficiency (PCE) and other related parameters were calculated using the formulas as reported in refs [32,33] and a detailed comparison of cell parameters is presented inTable As earlier reports on the OPV have proven, the active area and active layer thickness is directly related to the power conversion efficiency (PCE) [34] The assembly of the organic photovoltaics based P3HT:PC70BM that was utilized in this research is shown inFig 4(aeb) We have tried a possible modification in the conventional architecture of [35] P3HT:PC70BM solar cell by a successful incorporation of precisely synthesized pure CuO nanoparticles The possible band alignment of pristine P3HT:PC70BM blend and CuO incorporated P3HT:PC70BM ternary blend are presented inFig 4(ced) and are well supported by the available literature [35] Short circuit current density versus open circuit voltage (J-V) characterization (Fig 4e) of pristine P3HT:PC70BM solar cell has been achieved with an ~2.85% efficiency FromTable 1, it is obvious that after the incorporation of CuO-NPs, Jsc increased from 9.43 mA/cm2to 11.32 mA/cm2 This indicates that the properties of the CuO-NPs affect the Jscof the device as well Device parameters such as Jsc,Voc, and FF show increasing behavior up to a certain (5 wt%) composition and then decrease beyond this concentration The power conversion effi -ciency follows the same trend, increasing from 2.85% to 3.82% and then decreasing with further addition of CuO which may be due to a higher aggregation of the CuO [8] The aggregates let the solar cell structure collapse and remove the network for charge collection

Wanninayake et al (2015) reported on the P3HT:PCBM solar cell with CuO nanoparticles and obtained a value for the PCE of ~2.96% [26] In comparison with reported CuO incorporated P3HT:PC70BM solar cells, ourfindings are novel and better because of the utili-zation of a cost effective synthesis method for preparing CuO-NPs and by serving them as photo absorber for achieving enhanced power conversion efficiency Also, it is our belief, that this is the maximal reported PCE based on a CuO incorporated P3HT:PC70BM solar cell In respect to device architecture, it is the most desired approach for improving the absorption as well as Jscof the prepared devices Further, the obtained results were compared with the re-ported P3HT:CuO:PC70BM solar cell (normal configuration) values and are summarized inTable

The effect of CuO-NPs inclusion is fairly well observed in the series and shunt resistances as revealed fromFig S2 The series resistance (Rs) was 46Ufor pristine P3HT:PC70BM With an increase in the CuO-NPs concentration to 5.0 wt %, the series resistance (Rs) decreased to 11U Similarly, the maximal shunt resistance (Rsh) was observed for P3HT:CuO5wt%:PC70BM, indicating a reduced electronehole recombination rate and a leakage current due to the presence of CuO-NPs [36] The CuO-NPs may create a network which can efﬁciently dissociate the exciton which results in the

higher shunt resistance The shunt resistance (Rsh) falls for higher concentration of CuO-NPs

In order to study the light harvesting capabilities of pristine P3HT:PC70BM and CuO incorporated P3HT:CuO:PC70BM devices, external quantum efficiency (EQE) spectra have been recorded (Fig 4f) More photons absorbed in the active layer (P3HT:CuO:PC70BM) is one possible reason for the improved carrier generation The maximal efficiency of the EQE spectra shows the same trend as Jsc and PCE As expected, the cell P3HT:CuO5wt %:PC70BM exhibited an extended photocurrent onset and showed a marked improvement in EQE in the region of 400 nme750 nm, compared to those of remaining (0%, 1%, 3%, 7%, and 10 wt% of CuO nanoparticles) based P3HT:PC70BM devices The maximal EQE of Fig 3.(a) The X-ray diffraction patterns of pristine and CuO-NPs incorporated P3HT:PC70BMfilms (b) The UV-Vis absorption spectrum of pristine and CuO-NPs incorporated P3HT:PC70BMfilms, Inset enlarged x-axis in range 540e800 nm

Table

Comparative analysis of device parameters of CuO incorporated P3HT:PC70BM solar cell with pristine P3HT:PC70BM solar cell

Fabricated devices Voc(V) Jsc(mA/cm2) FF (%) PCE (%) EQE (%)

P3HT:PC70BM 0.56 9.43 54.01 2.85±0.02 38

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the P3HT:CuO5wt%:PC70BM device was 50% at 550 nm which is higher than the rest of the devices (Table 1) The higher absorption range from 400 nm to 750 nm for the P3HT:CuO5wt%:PCBM device followed the same trend as the EQE spectra and can be combined

with a similar variation of the absorption curve The integrated Jsc calculated from the EQE spectra (Fig Fig 4f) was slightly lower (around 2%) compared to the Jsc value measured in J-V characteristics and shows that the Jscvalues are more trusting We Fig 4.(a) Device structure (b) Schematic diagram of the device structure (c, d) Energy level diagram of the component materials used for device fabrication using Ref [23e25] (e) Current densityevoltage (JeV) characteristics of pristine and CuO-NPs incorporated P3HT:PC70BM devices (f) External quantum efﬁciency (EQE) and corresponding integral current of the pristine and CuO-NPs incorporated P3HT:PC70BM devices

Table

Few reports were found on CuO incorporated P3HT:CuO-NPs:PC70BM (Based on the Scopus data) till date with different conﬁguration (normal and inverted) of solar cells

Author CuO-NPs Cell Conﬁguration Type PCE Ref

E Salim CuO-NPs

Sigma Aldrich

ITO/ZnO/P3HT:CuO:PCBM/MoOx/Ag Inverted 4.1 25

M Ikram CuO-NPs

Sigma Aldrich

ITO/ZnO/(P3HT:CuO:PCBM/MoO3/Ag) Inverted 4.09 23

M Ikram CuO-NPs

Sigma Aldrich

ITO/ZnO/(P3HT:CuO:PCBM/MoO3/Ag) Inverted 3.7 24

A P Wanninayake CuO-NPsnanocs.comUSA ITO/PEDOT:PSS (with Au-NPs)/P3HT/PCBM/CuO/Al Normal 3.5 36

A P Wanninayake CuO-NPsnanocs.comUSA ITO/PEDOT:PSS/P3HT/PCBM/CuO-NP/Al Normal 2.9 26

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consider that the improvement in EQE and Jsc results from the effective light scattering Meanwhile, the FF value (56.76%) of the P3HT:CuO5wt%:PCBM device is high, indicating that the interface between the ITO/PEDOT:PSS and the active layer (P3HT:CuO5wt%:PCBM) keeps a respectable contact quality, which is also reﬂected by the Rsand Rshvalues

4 Conclusions

The present piece of work successfully fabricates P3HT:PC70BM solar cells by incorporating wet chemically synthesized CuO nanoparticles to adjust the morphology of the active layer by which a significant enhancement of the device efficiency is achieved It is innovative to adopt wet chemically synthesized CuO nanoparticles as an additive instead of the conventional organic high-boiling compound This is the novelty factor of this work A power con-version efficiency of ~2.85% has been achieved for pristine P3HT:PC70BM solar cells However, a higher power conversion ef-ficiency of 3.82% is effectively achieved for an optimal amount of CuO-NPs added P3HT:PC70BM because of an efficient excitation generation, better light absorption and a photoexcited charge separation and collection It is inferred that the incorporation of CuO nanoparticles into the P3HT:PC70BM blend can efficiently enhance the device performance which is validated by the EQE study as well Additionally, the shift in the absorption spectrum to the visible region would help in a better absorption of light after the incorporation of CuO-NPs in the P3HT:PC70BM blend Such sort of research paves the way to design an easy route for the synthesis of copper oxide nanoparticles Also, P3HT:PC70BM with an enhanced efficiency may be useful for further optoelectronic applications

Declaration of Competing Interest

The authors declare that they have no conﬂict of interests

Acknowledgments

HS is thankful to UGC, New Delhi, India and MPCST Bhopal for the award of MANF (F1-17.1/2011-12/MANF-MUS-MAD-4694) and FTYS (File No: 83/CST/FTYS/2016) MRP acknowledges CSIR, New Delhi for the award of SRF (ack no 163320/2K14/1) Authors would like to thank Director CSIR-NCL, Pune, and are pleased to acknowledge Dr K Krishnamoorthy, Scientist, Polymers and Advanced Materials Laboratory, CSIR NCL, Pune for solar cell fabrication and testing The help rendered by Mr S Chithiravel is highly appreciated Authors are thankful to the Director-UGC-DAE-CSR, Indore Centre for performing material characterization and grateful to Dr R J Choudhary for providing the XPS facility In addition, authors acknowledge Mr Wadikar and Mr Sharad Kumar (AIPES, Beamline BL-2 Indus-1, RRCAT, Indore) for technical assistance

Appendix A Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2020.01.004

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