Role of balanced charge carrier transport in low band gap polymer:Fullerene bulk heterojunction solar cells

Lowering of the optical band gap of conjugated polymers in bulk heterojunction solar cells not only leads to an increased absorption but also to an increase of the optimal active layer thickness due to interference effects at longer wavelengths. The increased carrier densities due to the enhanced absorption and thicker active layers make low band gap solar cells more sensitive to formation of space charges and recombination. By systematically red shifting the optical parameters of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylenevinylene] and 6,6‐phenyl C₆₁‐butyric acid methyl ester, we simulate the effect of a reduced band gap on the solar cell efficiencies. We show that especially the fill factor of low band gap cells is very sensitive to the balance of the charge transport. For a low band gap cell with an active layer thickness of 250 nm, the fill factor of 50% for balanced transport is reduced to less than 40% by an imbalance of only one order of magnitude. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Synthesis and photovoltaic properties of a low‐band‐gap polymer consisting of alternating thiophene and benzothiadiazole derivatives for bulk‐heterojunction and dye‐sensitized solar cells

We synthesized a novel low‐band‐gap, conjugated polymer, poly[4,7‐bis(3′,3′‐diheptyl‐3,4‐propylenedioxythienyl)‐2,1,3‐benzothiadiazole] [poly(heptyl₄‐PTBT)], consisting of alternating electron‐rich, diheptyl‐substituted propylene dioxythiophene and electron‐deficient 2,1,3‐benzothiadiazole units, and its photovoltaic properties were investigated. A thin film of poly(heptyl₄‐PTBT) exhibited an optical band gap of 1.55 eV. A bulk‐heterojunction solar cell with indium tin oxide/poly(3,4‐ethylenedioxythiophene)/poly(heptyl₄‐PTBT): methanofullerene [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) (1:4)/LiF/Al was fabricated with poly(heptyl₄‐PTBT) as an electron donor and PCBM as an electron acceptor and showed an open‐circuit voltage, short‐circuit current density, and power conversion efficiency of 0.37 V, 3.15 mA/cm², and 0.35% under air mass 1.5 (AM1.5G) illumination (100 mW/cm²), respectively. A solid‐state, dye‐sensitized solar cell with a SnO₂:F/TiO₂/N3 dye/poly(heptyl₄‐PTBT)/Pt device was fabricated with poly(heptyl₄‐PTBT) as a hole‐transport material. This device exhibited a high power conversion efficiency of 3.1%, which is the highest power conversion efficiency value with hole‐transport materials in dye‐sensitized solar cells to date. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1394–1402, 2007     

Conjugated polymer‐functionalized graphite oxide sheets thin films for enhanced photovoltaic properties of polymer solar cells

In this study, the maleimide‐thiophene copolymer‐functionalized graphite oxide sheets (PTM21‐GOS) and carbon nanotubes (PTM21‐CNT) were developed for polymer solar cell (PSC) applications. The grafting of PTM21‐OH onto the CNT and GO sheets was confirmed using FTIR spectroscopy. PTM21‐CNT and PTM21‐GOS exhibited excellent dispersal behavior in organic solvents. Better thermal stability was observed for PTM21‐CNT and PTM21‐GOS as compared with that for PTM21‐OH. In addition, the optical band gaps of PTM21‐GOS and PTM21‐CNT were lower than that of PTM21‐OH. We incorporated PTM21‐GOS and PTM21‐CNT individually into poly(3‐hexylthiophene) (P3HT)/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) blends for use as photoconversion layers of PSCs. Good distributional homogeneity was observed for PTM21‐GOS or PTM21‐CNT in the P3HT/PCBM blend film. The UV–vis absorption peaks of the blend films red‐shifted slightly upon increasing the content of PTM21‐GOS or PTM21‐CNT. The band gap energies and LUMO/HOMO energy levels of the P3HT/PTM21‐GOS and P3HT/PTM21‐CNT blend films were slightly lower than those of the P3HT film. The conjugated polymer‐functionalized PTM21‐GOS and PTM21‐CNT behaved as efficient electron acceptors and as charge‐transport assisters when incorporated into the photoactive layers of the PSCs. PV performance of the PSCs was enhanced after incorporating PTM21‐GOS or PTM21‐CNT in the P3HT/PCBM blend. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

All-polymer solar cells utilizing low band gap polymers as donor and acceptor

All-polymer solar cells based on blends of the low band gap polymers poly{[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]} (PTB7) and poly{[N,N-9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} (P(NDI2OD-T2)) are demonstrated. The use of the donor polymer PTB7 instead of poly(3-hexylthiophene) results in a higher open-circuit voltage and an overall spectral response better matched to the solar spectrum. A power conversion efficiency of 1.1% is reported with a peak external quantum efficiency of 18% at a wavelength of 680 nm. The microstructure of PTB7:P(NDI2OD-T2) blends is also investigated using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS), near-edge X-ray fine-structure (NEXAFS) spectroscopy, atomic force microscopy (AFM), and scanning transmission X-ray microscopy (STXM). GIWAXS measurements show that PTB7:P(NDI2OD-T2) blends contain P(NDI2OD-T2) crystallites with a (100) thickness of 9.5 nm dispersed in an amorphous PTB7 matrix. STXM measurements indicate a lack of mesoscale phase separation, with AFM and NEXAFS measurements revealing a P(NDI2OD-T2)-rich top surface with fibrillar morphology. These results indicate that the pairing of low band gap polymers as both donor and acceptor polymers in all-polymer solar cells may be an effective strategy for realizing high-efficiency all-polymer solar cells. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Study of spatial inhomogeneity in inverted all‐polymer solar cells: Effect of solvent and annealing

The efficiency optimization of bulk heterojunction solar cells requires the control of the local active materials arrangement in order to obtain the best compromise between efficient charge generation and charge collection. Here, we investigate the large scale (10–100 μm) inhomogeneity of the photoluminescence (PL) and the external quantum efficiency (EQE) in inverted all‐polymer solar cells (APSC) with regioregular poly(3‐hexylthiophene) (P3HT):poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) active blends. The morphology and the local active polymer mixing are changed by depositing the active layer from four different solvents and by thermal annealing. The simultaneous PL and EQE mapping allowed us to inspect the effects of local irregularities of active layer thickness, polymer mixing, polymer aggregation on the charge generation and collection efficiencies. In particular, we show that the increase of the solvent boiling point affects the EQE non‐uniformity due to thickness fluctuations, the density non‐uniformity of rrP3HT aggregate phase, and the blend components clustering. The thermal annealing leads to a general improvement of EQE and to an F8BT clustering in all the samples with locally decrease of the EQE. We estimate that the film uniformity optimization can lead to a total EQE improvement between 2.7 and 6.3 times. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 804–813     

Benzotriazole and benzodithiophene containing medium band gap polymer for bulk heterojunction polymer solar cell applications

An alternating donor‐acceptor copolymer based on a benzotriazole and benzodithiophene was synthesized and selenophene was incorporated as π‐bridge. The photovoltaic and optical properties of polymer were studied. The copolymer showed medium band gap and dual absorption peaks in UV‐Vis absorption spectra. Photovoltaic properties of P‐SBTBDT were performed by conventional device structure. The OSC device based on polymer: PC₇₁BM (1:1, w/w) exhibited the best PCE of 3.60% with a V_oc of 0.67 V, a J_sc of 8.95 mA/cm², and a FF of 60%. This finding was supported with morphological data and space charge limited current (SCLC) mobilities. The hole mobility of the copolymer was estimated through SCLC model. Although surface roughness of the active layer is really high, mobility of a polymer was found as 7.46 × 10^?3 cm²/Vs for optimized device that can be attributed to Se?Se interactions due to the larger, more‐polarizable Se atom. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 528–535     

Synthesis and characterization of a new phenanthrenequinoxaline‐based polymer for organic solar cells

Two donor–acceptor (D‐A) conjugated polymers, PQx and PphQx, composed of alkylthienyl‐substituted benzo[1,2‐b:4,5‐b']dithiophene (BDTT) as the electron donor and the new electron acceptors quinoxaline (Qx) or phenanthrenequinoxaline (phQx), were synthesized with Stille cross‐coupling reactions. The number‐averaged molecular weights (M_n) of PQx and PphQx were found to be 25.1 and 23.2 kDa, respectively, with a dispersity of 2.6. The band‐gap energies of PQx and PphQx are 1.82 and 1.75 eV, respectively. These results indicate that, because phQx units have highly planar structures, their inclusion in D‐A polymers will be a very effective method for increasing the polymers' effective conjugation lengths. The hole mobilities of PQx and PphQx were determined to be 5.0 × 10^?5 and 2.2 × 10^?4 cm² V^?1 s^?1, respectively. A polymer solar cell device prepared with PphQx as the active layer was found to exhibit a power conversion efficiency (PCE) of 5.03%; thus, the introduction of phQx units enhanced both the short circuit current density and PCE of the device. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2804–2810     

Low band‐gap D–A conjugated copolymers based on anthradithiophene and diketopyrrolopyrrole for polymer solar cells and field‐effect transistors

Two conjugated copolymers PADT‐DPP and PADT‐FDPP based on anthradithiophene and diketopyrrolopyrrole, with thiophene and furan as the π‐conjugated bridge, respectively, were successfully synthesized and characterized. The number‐averaged molecular weights of the two polymers are 38.7 and 30.2 kg/mol, respectively. Polymers PADT‐DPP and PADT‐FDPP exhibit broad absorption bands and their optical band gaps are 1.44 and 1.50 eV, respectively. The highest occupied molecular orbital energy level of PADT‐DPP is located at ?5.03 eV while that of PADT‐FDPP is at ?5.16 eV. In field‐effect transistors, PADT‐DPP and PADT‐FDPP displayed hole mobilities of 4.7 × 10^?3 and 2.7 × 10^?3 cm²/(V s), respectively. In polymer solar cells, PADT‐DPP and PADT‐FDPP showed power conversion efficiency (PCE) of 3.44% and 0.29%, respectively. Atomic force microscopy revealed that the poor efficiency of PADT‐FDPP should be related to the large two‐phase separation in its active layer. If 1,8‐diiodooctane (DIO) was used as the solvent additive, the PCE of PADT‐DPP remained almost unchanged due to very limited morphology variation. However, the addition of DIO could remarkably elevate the PCE of PADT‐FDPP to 2.62% because of the greatly improved morphology. Our results suggest that the anthradithiophene as an electron‐donating polycyclic system is useful to construct new D–A alternating copolymers for efficient polymer solar cells. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1652–1661     

Diketopyrrolopyrrole‐based liquid crystalline conjugated donor–acceptor copolymers with reduced band gap for polymer solar cells

A new liquid crystalline (LC) acceptor monomer 2,5‐bis[4‐(4′‐cyanobiphenyloxy)dodecyl]‐3,6‐dithiophen‐2‐yl‐pyrrolo[3,4‐c]pyrrole‐1,4‐dione (TDPPcbp) was synthesized by incorporating cyanobiphenyl mesogens into diketopyrrolopyrrole (DPP). The monomer was copolymerized with bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′] dithiophene (BDT) and N‐9′‐heptadecanylcarbazole (CB) donors to obtain donor–acceptor alternating copolymers poly[4,8‐bis(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐3,6‐bis(thiophen‐5‐yl)‐2,5‐bis[4‐(4′‐cyanobiphenyloxy)dodecyl]‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione] (PBDTDPPcbp) and poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐3,6‐bis(thiophen‐5‐yl)‐2,5‐bis[4‐(4′‐cyano‐biphenyloxy)dodecyl]‐2,5‐dihydropyrrolo[3, 4‐c]pyrrole‐1,4‐dione] (PCBTDPPcpb) with reduced band gap, respectively. The LC properties of the copolymers, the effects of main chain variation on molecular packing, optical properties, and energy levels were analyzed. Incorporating the mesogen cyanobiphenyl units not only help polymer donors to pack well through mesogen self‐organization but also push the fullerene acceptor to form optimized phase separation. The bulk heterojunction photovoltaicdevicesshow enhanced performance of 1.3% for PBDTDPPcbp and 1.2% for PCBTDPPcbp after thermal annealing. The results indicate that mesogen‐controlled self‐organization is an efficient approach to develop well‐defined morphology and to improve the device performance. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Synthesis and characterization of fluorene‐based low‐band gap copolymers containing propylenedioxythiophene and benzothiadiazole derivatives for bulk heterojunction photovoltaic cell applications

The following noble series of soluble π‐conjugated statistical copolymers was synthesized by palladium catalyzed Suzuki polymerization: poly[(9,9‐dioctylfluorene)‐alt‐(4,7‐bis(3′,3′‐dihepyl‐3,4‐propylenedioxythienyl)‐2,1,3‐benzothiadiazole)] (PFO‐PTBT) derived from poly(9,9‐dioctylfluorene) (PFO) and poly[(4,7‐bis(3′,3′‐dihepyl‐3,4‐propylenedioxythienyl)‐2,1,3‐benzothiadiazole)] poly(heptyl₄‐PTBT). The structure and properties of these polymers were characterized using ¹H‐, ¹³C‐NMR, UV–visible spectroscopy, elemental analysis, GPC, DSC, TGA, photoluminescence (PL), and cyclic voltammetry (CV). The statistical copolymers, PFO‐PTBT (9:1, 8.4:1.6, 6.5:3.5), were soluble in common organic solvents and easily spin coated onto indium‐tin oxide (ITO) coated glass substrates. The weight‐average molecular weight (M_w) and polydispersity of the PFO‐PTBT ranged from (1.0–4.2) × 10⁴ and 1.5–2.3, respectively. Bulk heterojunction photovoltaic cells with an ITO/PEDOT/PFO‐PTBT:PCBM/LiF/Al configuration were fabricated, and the devices using PFOPTBT (6.5:3.5) showed the best performance compared with those using PFO‐PTBT (9:1, 8.4:1.6). A maximum power conversion efficiency (PCE) of 0.50% (V_oc = 0.66 V, FF = 0.29) was achieved with PFO‐PTBT (6.5:3.5). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6175–6184, 2008     

Crystalline and active additive for optimization morphology and absorption of narrow bandgap polymer solar cells

The morphology of active layer with an interpenetrating network structure and appropriate phase separation is of great significance to improve the photovoltaic performance for polymer solar cells. A highly crystalline small molecule named DPP-TP6 was synthesized and incorporated into the narrow bandgap polymer solar cells to optimize the morphology of PTB7:PC₇₁BM active layer. The DPP-TP6 small molecule was demonstrated to enhance the light absorbance of active layer and play the role of energy cascade to increase the exciton separation and charge transfer. What's more, DPP-TP6 facilitated forming interpenetrating network structure and increasing the phase separation size of ternary blends. These phenomena lead to a higher hole mobility and a more balanced carrier mobility, so as to increase the power conversion efficiency to 7.85% at DPP-TP6 weight ratio of 8 wt %, comparing to the pristine PTB7:PC₇₁BM system of 6.50%. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 726–733     

Influence of the processing solvent on the photoactive layer nanomorphology of P3HT/PC60BM solar cells

In this article, it is demonstrated that doctor blading of thin poly‐3‐hexylthiophene/phenyl‐C₆₁‐butyric acid methyl ester (1/1) bulk‐hetero junction films from toluene leads to an improved nanocrystallinity, when compared with their unannealed chlorobenzene processed counterparts. This difference in morphology was demonstrated by solid‐state NMR and Rapid Heating Cooling Calorimetry (RHC), being useful complementary techniques to investigate the active layer morphology of photovoltaic devices. An increased PC₆₀BM nanocrystallinity is indicated by several NMR relaxation decay times (T_1C, T_1H, and T_1ρH) and confirmed by an increase of the melting enthalpy in RHC experiments. An improved solar cell performance further strengthens this conclusion. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

聚合物太阳能电池材料研究进展

本文介绍了几种常见的聚合物太阳电池材料。综述了聚合物太阳电池材料的合成、发展历史和现状,对其应用前景进行了展望。参考文献59篇。     

Influence of fullerene photodimerization on the PCBM crystallization in polymer: Fullerene bulk heterojunctions under thermal stress

For an increased lifetime of polymer:fullerene bulk heterojunction (BHJ) solar cells, an understanding of the chemical and morphological degradation phenomena taking place under operational conditions is crucial. Phase separation between polymer and fullerene induced by thermal stress has been pointed out as a major issue to overcome. While often the effect of thermal stress on the morphology of polymer:fullerene BHJ is investigated in the darkness, here we observe that light exposure slows down fullerene crystallization and phase separation induced at elevated temperatures. The observed photo‐stabilizing effect on active layer morphology is quite independent on the polymer and is attributed to light‐induced dimerization of the fullerene. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1209–1214     

Toward low‐band gap dithienophosphole copolymers for an application in organic solar cells

Two functionalized dithieno[3,2‐b:2′,3′‐d]phospholes with solubilizing groups have been synthesized that allow for the generation of a series of π‐conjugated AB‐ and ABC‐copolymers. The polymers obtained show notable optoelectronic properties with red‐shifted absorption and emission in the orange to red section of the optical solar spectrum. Although combination of dithienophosphole units with fluorene building blocks gives access to processable polymers with band gaps between 2.2 and 2.3 eV in solution and 2.0 eV in the solid state, an ABC copolymer based on dithienophosphole, fluorene, and bis(thienyl)benzothiadiazole units was found to not only exhibit a suitable band gap for solar cell applications (solution: 2.0 eV; solid state: 1.7 eV) but also showed good solubility as well as good electron transfer properties in the presence of fullerene (C₆₀). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8179–8190, 2008     

Stabilization of the film morphology in polymer: Fullerene heterojunction solar cells with photocrosslinkable bromine‐functionalized low‐bandgap copolymers

Novel bromine‐functionalized photocrosslinkable low‐bandgap copolymers, PBDTTT‐Br25 and PBDTTT‐Br50, are synthesized via Stille cross‐coupling polymerization for the purpose of stabilizing the film morphology in polymer solar cells (PSCs). Photocrosslinking of PBDTTT‐Br25 and PBDTTT‐Br50 copolymers dramatically improves the solvent resistance of the active layer without disrupting the molecular ordering and charge transport, which is confirmed by the insolubility of the films washed by organic solvents and by their thermal behavior. As a result, the formation of large aggregations of fullerene is suppressed in polymer:fullerene blend films even after prolonged thermal annealing, and the stability of the device is enhanced when compared with cells based on noncrosslinkable PBDTTT. The power conversion efficiency of the PSCs based on PBDTTT‐Br25 and PBDTTT‐Br50 reaches 5.17% and 4.48%, respectively, which is improved obviously in comparison with that (4.26%) of the PSCs based on the control polymer PBDTTT. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3123–3131     

Understanding the crystallization process of a diketopyrrolopyrrole-based conjugated polymer in blend films

The film morphology of fullerene and diketopyrrolopyrrole-based conjugated polymers (PDPPs) blends largely influences the device performance in organic solar cells. It is critical to control the morphology of blend films, which usually requires investigations of the crystallization of PDPP-based thin films. Here, we study the influence of marginal solvent additive 1,2-dichlorobenzene (ODCB) and non-solvent additive 1,8-diiodooctane (DIO) on the crystallization of poly[2,5-bis(2-octyldodecyl)pyrrolo-[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-2,2′: 5′,2″: 5″,2″′-quaterthiophene] (PDQT). The blends formed fibril structures in thin films, as revealed by transmission electron microscopy. The fibril density increased and the width decreased with the ODCB amount. The critical ODCB content to achieve constant fibril width is almost proportional to the concentration of PDQT. Higher ODCB content also results in higher fibril density in pure PDQT films. In contrast, the amount of DIO has a negligible influence on the fibril width and density of thin films. Moreover, novel dendritic fibrils were formed in PDQT films upon addition of ODCB. A model based on nucleation and growth is proposed to explain these findings. The heterogeneous nucleation was dominant with the presence of ODCB, while the homogeneous nucleation was prevailing when DIO was used. The results show that initial nucleation density and growth direction are key factors determining the fibril width.     

Decreased domain size and improved crystallinity by adjusting solvent–polymer interaction parameters in all‐polymer solar cells

We have investigated the effect of solvent–polymer interaction on the morphology, crystallinity, and device performance of poly‐(3‐hexylthiophene) (P3HT) and poly{2,7‐(9,9‐didodecyl‐fluorene)‐alt‐5,5‐[4′,7′‐bis(2‐thienyl)‐2′,1′,3′‐benzothia‐diaole]} (PF12TBT) blend system. 3‐Hexylthiophene (3‐HT), which had the similar structural units with both donor and acceptor materials, was chosen as the solvent additive to be added into the main solvent chlorobenzene (CB), to adjust the solvent–polymer interaction. With the 3‐HT percentage increasing from 5 to 30% in CB solution, the solvent–polymer interaction between polymer and solvent molecules decreased slightly according to the calculated solubility parameters (δ) and interaction parameters (χ₁₂). As a result, nanoscale phase‐separated and interconnected morphology with decreased domain size of both donor and acceptor was formed. Meanwhile, the order of P3HT molecule was enhanced which resulted from the extended film drying time and increased molecular planarity after incorporation of 3‐HT. The power conversion efficiency (PCE) had a gradual improvement to 1.08% as the 3‐HT percentage reached 10%, which can be attributed to the enhanced short‐circuit current (J_sc) and fill factor (FF). However, when the 3‐HT percentage exceeded 20%, the decreased J_sc and FF ultimately decreased the PCE. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 288–296