Iordania Constantinou
North Carolina State University
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Featured researches published by Iordania Constantinou.
ACS Applied Materials & Interfaces | 2015
Iordania Constantinou; Tzung-Han Lai; Dewei Zhao; Erik Klump; James J. Deininger; Chi Kin Lo; John R. Reynolds; Franky So
The effect of air processing, with air exposure varying from minutes to hours prior to encapsulation, on photovoltaic device performance has been studied through a series of electrical characterizations and optical simulations for a donor/acceptor polymer-based organic solar cell based on poly(dithienogermole-alt-thienopyrrolodione) p(DTG-TPD)/PC71BM blends. A ∼10% degradation in power conversion efficiency was observed due to air processing with 10 min exposure time, with AM1.5 power conversion efficiencies (PCEs) decreasing from 8.5 ± 0.25% for devices processed in inert nitrogen atmosphere to 7.7 ± 0.18% for devices processed in ambient air. After 3 h air exposure, the PCE leveled off at 7.04 ± 0.1%. This decrease is attributed partially to interface issues caused by exposure of the electrode materials to oxygen and water and partially to a degradation of the hole transport in the active layer.
ACS Applied Materials & Interfaces | 2015
Iordania Constantinou; Tzung-Han Lai; Erik Klump; Subhadip Goswami; Kirk S. Schanze; Franky So
The effect of polymer side chains on device performance was investigated for PBDT(EtHex)-TPD(Oct):PC70BM and PBDT(EtHex)-TPD(EtHex):PC70BM BHJ solar cells. Going from a linear side chain on the polymers acceptor moiety to a branched side chain was determined to have a negative impact on the overall device efficiency, because of significantly reduced short-circuit current (J(sc)) and fill factor (FF) values. Sub-bandgap external quantum efficiency (EQE) and transient photoluminescence (PL) measurements showed more-efficient carrier generation for the polymer with linear side chains, because of a higher degree of charge-transfer (CT) state delocalization, leading to more-efficient exciton dissociation. Furthermore, the increase in π-π stacking distance and disorder for the bulkier ethylhexyl side chain were shown to result in a lower hole mobility, a higher bimolecular recombination, and a higher energetic disorder. The use of linear side chains on the polymers acceptor moiety was shown to promote photogeneration, because of more-effective CT states and favorable carrier transport resulting in improved solar cell performance.
Journal of Materials Chemistry | 2017
Nicole Bauer; Qianqian Zhang; Jingbo Zhao; Long Ye; Joo Hyun Kim; Iordania Constantinou; Liang Yan; Franky So; Harald Ade; He Henry Yan; Wei You
Non-fullerene acceptors (NFAs) are becoming a serious contender to fullerene-based electron acceptors in organic photovoltaics, due to their structural versatility and easily tunable optical and electronic properties. However, NFA-based solar cells often have a decreased short-circuit current (Jsc) and fill factor (FF) compared to their fullerene-based counterparts. Here, we investigate the fundamental causes of this decrease in the performance of solar cells using a non-fullerene acceptor (SF-PDI2) paired with two polymer donors, FTAZ and PyCNTAZ, compared with their fullerene-based counterparts. Through a number of experimental techniques and morphological studies, we show that the SF-PDI2-based solar cells suffer from insufficient charge generation, transport, and collection when compared with the PCBM-based solar cells. The SF-PDI2-based solar cells show increased bimolecular recombination, which, together with other recombination loss mechanisms in these cells, causes a significant decrease in their Jsc and FF. Notably, the less pure domains, low electron mobility (on the order of 10−5 cm2 V−1 s−1), and imbalanced mobility (in regard to the hole mobility) further explain the low FF. On the other hand, the higher open-circuit voltage (Voc) in the SF-PDI2 devices is mainly due to the increase in the CT state energy. It is worth mentioning that the PyCNTAZ-based devices show an ultralow charge separation energy (ΔECS), close to 0 eV. Our results demonstrate that further increasing the mobility (both of electrons and holes) in these NFA-based solar cells would be a viable approach to further enhance the efficiency of these new types of solar cells, ideally, without losing the high Voc of such cells.
Journal of Materials Chemistry | 2017
Joo Hyun Kim; Abay Gadisa; Charley Schaefer; Huifeng Yao; Bhoj Gautam; Nrup Balar; Masoud Ghasemi; Iordania Constantinou; Franky So; Brendan T. O'Connor; Kenan Gundogdu; Jianhui Hou; Harald Ade
The performance of polymer–fullerene bulk heterojunction (BHJ) solar cells is highly affected by the morphology of the blend film. Though the structure of the BHJ morphology is well-understood, the relationship between relevant material interactions and BHJ morphological evolutions is poorly understood and seldom explored. In this report, we discuss the impact of polymer molecular weight (MW) on thermodynamic and kinetic phenomena which have a drastic influence on the nanoscale BHJ morphology. The blend film comprises a highly aggregating low bandgap diketopyrrolopyrrole-based polymer PDPP3T and the fullerene electron acceptor molecule PC71BM, cast from a 1,2-dichlorobenzene (DCB) solution with and without the additive 1,8-diiodooctane (DIO). The pair-wise interactions among the components were evaluated by Flory–Huggins interaction parameters (χ). The BHJ blend of PDPP3T and PC71BM exhibited liquid–liquid (L–L) phase separation as a result of strong polymer–fullerene interactions (large χPDPP3T–fullerene) in DCB solution. In contrast, addition of 3% DIO into the DCB solution is found to stimulate polymer aggregation which gives rise to liquid–solid (L–S) phase separation. Large χPDPP3T–fullerene and χPDPP3T–solvent were observed as a result of increasing polymer MW, and these changes promote strong L–L phase separation and polymer aggregation in the blend solution, respectively. The latter interactions have led to low molecular mobility, with an end result of reduced crystallinity and smaller domain size of the BHJ films. The observed dramatic MW-dependent morphological changes were also manifested in solar cell outputs as well as charge carrier dynamics calculated by transient absorption measurements.
Advanced Materials | 2018
Bhoj Gautam; Erik Klump; Xueping Yi; Iordania Constantinou; Nathan T. Shewmon; Amin Salehi; Chi Kin Lo; Zilong Zheng; Jean-Luc Brédas; Kenan Gundogdu; John R. Reynolds; Franky So
Interfaces between donor and acceptor in a polymer solar cell play a crucial role in exciton dissociation and charge photogeneration. While the importance of charge transfer (CT) excitons for free carrier generation is intensively studied, the effect of blending on the nature of the polymer excitons in relation to the blend nanomorphology remains largely unexplored. In this work, electroabsorption (EA) spectroscopy is used to study the excited-state polarizability of polymer excitons in several polymer:fullerene blend systems, and it is found that excited-state polarizability of polymer excitons in the blends is a strong function of blend nanomorphology. The increase in excited-state polarizability with decreased domain size indicates that intermixing of states at the interface between the donor polymers and fullerene increases the exciton delocalization, resulting in an increase in exciton dissociation efficiency. This conclusion is further supported by transient absorption spectroscopy and time-resolved photoluminescence measurements, along with the results from time-dependent density functional theory calculations. These findings indicate that polymer excited-state polarizability is a key parameter for efficient free carrier generation and should be considered in the design and development of high-performance polymer solar cells.
Advanced Energy Materials | 2014
Harrison Ka Hin Lee; Zhao Li; Iordania Constantinou; Franky So; Sai-Wing Tsang; S. K. So
Advanced electronic materials | 2015
Iordania Constantinou; Tzung-Han Lai; Hsien-Yi Hsu; Sin-Hang Cheung; Erik Klump; Kirk S. Schanze; S. K. So; Franky So
Advanced Energy Materials | 2017
Iordania Constantinou; Xueping Yi; Nathan T. Shewmon; Erik Klump; Cheng Peng; Sofia Garakyaraghi; Chi Kin Lo; John R. Reynolds; Felix N. Castellano; Franky So
Chemistry of Materials | 2016
Tzung-Han Lai; Iordania Constantinou; Caroline Grand; Erik Klump; Sujin Baek; Hsien-Yi Hsu; Sai-Wing Tsang; Kirk S. Schanze; John R. Reynolds; Franky So
ACS Applied Materials & Interfaces | 2016
Nathan T. Shewmon; Hyeonggeun Yu; Iordania Constantinou; Erik Klump; Franky So