Predicting thermal stability of organic solar cells through an easy and fast capacitance measurement

Degradation of organic photovoltaic (OPV) devices is currently a topic under intense research as it is one of the main limitations towards the commercialization of this technology. Morphological changes at both active layer and interfaces with the outer contacts are believed to determine main key issues to be overcome. In-line techniques are essential to rule out any effect arising during sample fabrication. Unfortunately, the number of physical techniques able to provide morphological information on complete and operational devices is certainly limited. In this work, we study the thermal degradation of bulk heterojunction (BHJ) solar cells composed by different donor polymers with techniques developed to provide in-situ information on operational devices. Capacitance measurement as a function of temperature monitors the electrical integrity of the active layer and provides the threshold temperature (TMAX) at which the whole device becomes thermally unstable. We found a direct correlation between the threshold temperature TMAX, obtained by capacitance–temperature measurements on complete OPV devices, and the power conversion efficiency decay measured at 85 °C. Devices tend to be thermally stable when the temperature of the thermal stress is below TMAX, while above TMAX evident changes in the active layer or at the active layer/electrode interface are also detected by confocal fluorescence microscopy. The capacitance method gives precious guidelines to predict the thermal stability of BHJ solar cells using an accelerated and easy test.

[1]  J. Bisquert,et al.  How the charge-neutrality level of interface states controls energy level alignment in cathode contacts of organic bulk-heterojunction solar cells. , 2012, ACS nano.

[2]  Yang Yang,et al.  An Efficient Triple‐Junction Polymer Solar Cell Having a Power Conversion Efficiency Exceeding 11% , 2014, Advanced materials.

[3]  A. Facchetti,et al.  Bithiophene Imide and Benzodithiophene Copolymers for Efficient Inverted Polymer Solar Cells , 2012, Advanced materials.

[4]  Jean Manca,et al.  Enhanced intrinsic stability of the bulk heterojunction active layer blend of polymer solar cells by varying the polymer side chain pattern , 2014 .

[5]  Kion Norrman,et al.  Water-induced degradation of polymer solar cells studied by H2(18)O labeling. , 2009, ACS applied materials & interfaces.

[6]  G. Wantz,et al.  Controlling the morphology and performance of bulk heterojunctions in solar cells. Lessons learned from the benchmark poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester system. , 2013, Chemical reviews.

[7]  Dramatic enhancement of fullerene anion formation in polymer solar cells by thermal annealing: Direct observation by electron spin resonance , 2014 .

[8]  A. Salleo,et al.  The Mechanism of Burn‐in Loss in a High Efficiency Polymer Solar Cell , 2012, Advanced materials.

[9]  M. Muccini,et al.  Fine Structural Tuning of Cyanated Dithieno[3,2-b:2′,3′-d]silole–Oligothiophene Copolymers: Synthesis, Characterization, and Photovoltaic Response , 2013 .

[10]  Chain‐Shu Hsu,et al.  Synthesis of conjugated polymers for organic solar cell applications. , 2009, Chemical reviews.

[11]  David G Lidzey,et al.  Competition between substrate-mediated π-π stacking and surface-mediated Tg depression in ultrathin conjugated polymer films , 2012, The European physical journal. E, Soft matter.

[12]  Suren A. Gevorgyan,et al.  Consensus stability testing protocols for organic photovoltaic materials and devices , 2011 .

[13]  Chuan Yi Tang,et al.  A 2.|E|-Bit Distributed Algorithm for the Directed Euler Trail Problem , 1993, Inf. Process. Lett..

[14]  Thanh Luan Nguyen,et al.  Enhanced Efficiency of Single and Tandem Organic Solar Cells Incorporating a Diketopyrrolopyrrole‐Based Low‐Bandgap Polymer by Utilizing Combined ZnO/Polyelectrolyte Electron‐Transport Layers , 2013, Advanced materials.

[15]  Tao Wang,et al.  Evolution of Structure, Optoelectronic Properties, and Device Performance of Polythiophene:Fullerene Solar Cells During Thermal Annealing , 2011 .

[16]  Wei You,et al.  Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer-fullerene solar cells. , 2011, Journal of the American Chemical Society.

[17]  Mm Martijn Wienk,et al.  Thermal Stability of Poly[2-methoxy-5-(2′-phenylethoxy)-1,4-phenylenevinylene] (MPE-PPV):Fullerene Bulk Heterojunction Solar Cells , 2011 .

[18]  M. Muccini,et al.  Influence of Incorporating Different Electron-Rich Thiophene-Based Units on the Photovoltaic Properties of Isoindigo-Based Conjugated Polymers: An Experimental and DFT Study , 2013 .

[19]  Germà Garcia-Belmonte,et al.  Polymer defect states modulate open-circuit voltage in bulk-heterojunction solar cells , 2013 .

[20]  Xiaoniu Yang,et al.  Effect of spatial confinement on the morphology evolution of thin poly(p-phenylenevinylene)/methanofullerene composite films , 2005 .

[21]  J. Bisquert,et al.  Shelf Life Degradation of Bulk Heterojunction Solar Cells: Intrinsic Evolution of Charge Transfer Complex , 2015 .

[22]  Jean Manca,et al.  Phase diagram of P3HT/PCBM blends and its implication for the stability of morphology. , 2009, The journal of physical chemistry. B.

[23]  Jan D'Haen,et al.  Modeling the temperature induced degradation kinetics of the short circuit current in organic bulk heterojunction solar cells , 2010 .

[24]  Agnès Rivaton,et al.  Light-induced degradation of the P3HT-based solar cells active layer , 2011 .

[25]  Alberto Salleo,et al.  Toward bulk heterojunction polymer solar cells with thermally stable active layer morphology , 2014 .

[26]  Etienne Goovaerts,et al.  Effect of temperature on the morphological and photovoltaic stability of bulk heterojunction polymer: fullerene solar cells , 2008 .

[27]  M. Muccini,et al.  Efficiency enhancement of P3HT:PCBM solar cells containing scattering Zn-Al hydrotalcite nanoparticles in the PEDOT:PSS layer , 2013 .

[28]  Yang Yang,et al.  A Selenium‐Substituted Low‐Bandgap Polymer with Versatile Photovoltaic Applications , 2013, Advanced materials.

[29]  Christopher R. McNeill,et al.  Morphology of all-polymer solar cells , 2012 .

[30]  Robert A. Riggleman,et al.  Influence of Backbone Rigidity on Nanoscale Confinement Effects in Model Glass-Forming Polymers , 2013 .

[31]  Luping Yu,et al.  Overcoming efficiency challenges in organic solar cells: rational development of conjugated polymers , 2012 .

[32]  Tao Wang,et al.  Rationalizing Phase Transitions with Thermal Annealing Temperatures for P3HT:PCBM Organic Photovoltaic Devices , 2012 .

[33]  Katsuhiko Ariga,et al.  Layer-by-layer Nanoarchitectonics: Invention, Innovation, and Evolution , 2014 .

[34]  J. Bisquert,et al.  Nanoscale mapping by electron energy-loss spectroscopy reveals evolution of organic solar cell contact selectivity , 2015 .

[35]  P. Blom,et al.  Degradation mechanisms in organic photovoltaic devices , 2012 .

[36]  Yan Yao,et al.  Self-propagating molecular assemblies as interlayers for efficient inverted bulk-heterojunction solar cells. , 2010, Journal of the American Chemical Society.

[37]  N. Yu,et al.  Flat optics with designer metasurfaces. , 2014, Nature materials.

[38]  C. Brabec,et al.  Electron Barrier Formation at the Organic‐Back Contact Interface is the First Step in Thermal Degradation of Polymer Solar Cells , 2014 .

[39]  Van der Hammen.  EVOLUTION OF STRUCTURE , 1998 .

[40]  Mario Leclerc,et al.  A calorimetric study of the phase transitions in poly(3-hexylthiophene) , 1995 .

[41]  Alberto Salleo,et al.  Structural Factors That Affect the Performance of Organic Bulk Heterojunction Solar Cells , 2013 .

[42]  Alex K.-Y. Jen,et al.  Recent advances in solution-processed interfacial materials for efficient and stable polymer solar cells , 2012 .

[43]  Graeme P. Williams,et al.  The Photo‐Stability of Polymer Solar Cells: Contact Photo‐Degradation and the Benefits of Interfacial Layers , 2013 .

[44]  Danjie Liu,et al.  Deviations of the glass transition temperature in amorphous conjugated polymer thin films. , 2013, Physical review. E, Statistical, nonlinear, and soft matter physics.

[45]  S. Bauer,et al.  Real-time in-situ observation of morphological changes in organic bulk-heterojunction solar cells by means of capacitance measurements , 2011 .

[46]  Agnès Rivaton,et al.  The mechanism of photo- and thermooxidation of poly(3-hexylthiophene) (P3HT) reconsidered , 2009 .

[47]  M. J. Richardson,et al.  Derivation of Accurate Glass Transition Temperatures by Differential Scanning Calorimetry , 1975 .

[48]  Jang-Joo Kim,et al.  Real Time Investigation of the Interface between a P3HT:PCBM Layer and an Al Electrode during Thermal Annealing. , 2009, Macromolecular rapid communications.

[49]  Annalisa Bonfiglio,et al.  Influence of thermal ageing on the stability of polymer bulk heterojunction solar cells , 2007 .

[50]  F. Krebs,et al.  A roll-to-roll process to flexible polymer solar cells: model studies, manufacture and operational stability studies , 2009 .

[51]  G. Garcia‐Belmonte,et al.  Oxygen doping-induced photogeneration loss in P3HT:PCBM solar cells , 2012 .

[52]  Yu-Shan Cheng,et al.  Fullerene Derivative‐Doped Zinc Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low‐Bandgap Polymer (PTB7‐Th) for High Performance , 2013, Advanced materials.

[53]  C. Stafford,et al.  Influence of chain stiffness on thermal and mechanical properties of polymer thin films , 2011 .

[54]  M. Muccini,et al.  Structural tuning of quinoxaline-benzodithiophene copolymers via alkyl side chain manipulation: synthesis, characterization and photovoltaic properties , 2014 .

[55]  Garry Rumbles,et al.  Pathways for the degradation of organic photovoltaic P3HT:PCBM based devices , 2008 .

[56]  Miao Xu,et al.  Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure , 2012, Nature Photonics.

[57]  Shijun Jia,et al.  Polymer–Fullerene Bulk‐Heterojunction Solar Cells , 2009, Advanced materials.

[58]  Barry P Rand,et al.  Delocalization and dielectric screening of charge transfer states in organic photovoltaic cells , 2014, Nature Communications.

[59]  M. Muccini,et al.  2D π-conjugated benzo[1,2-b:4,5-b']dithiophene- and quinoxaline-based copolymers for photovoltaic applications , 2013 .

[60]  Jean Manca,et al.  Improved thermal stability of bulk heterojunctions based on side-chain functionalized poly(3-alkylthiophene) copolymers and PCBM , 2013 .

[61]  R. Zamboni,et al.  Integration of a silk fibroin based film as a luminescent down-shifting layer in ITO-free organic solar cells , 2014 .

[62]  F. Krebs,et al.  Stability/degradation of polymer solar cells , 2008 .

[63]  Tao Wang,et al.  Correlating Structure with Function in Thermally Annealed PCDTBT:PC70BM Photovoltaic Blends , 2012 .