Inverted-type Polymer Solar Cells Research Articles

Effect of electron-withdrawing fluorine and cyano substituents on photovoltaic properties of two-dimensional quinoxaline-based polymers

In this study, strong electron-withdrawing fluorine (F) and cyano (CN) substituents are selectively incorporated into the quinoxaline unit of two-dimensional (2D) D–A-type polymers to investigate their effects on the photovoltaic properties of the polymers. To construct the 2D polymeric structure, electron-donating benzodithiophene and methoxy-substituted triphenylamine are directly linked to the horizontal and vertical directions of the quinoxaline acceptor, respectively. After analyzing the structural, optical, and electrochemical properties of the resultant F- and CN-substituted polymers, labeled as PBCl-MTQF and PBCl-MTQCN, respectively, inverted-type polymer solar cells with a non-fullerene Y6 acceptor are fabricated to investigate the photovoltaic performances of the polymers. It is discovered that the maximum power conversion efficiency of PBCl-MTQF is 7.48%, whereas that of PBCl-MTQCN is limited to 3.52%. This significantly reduced PCE of the device based on PBCl-MTQCN is ascribed to the formation of irregular, large aggregates in the active layer, which can readily aggravate the charge recombination and charge transport kinetics of the device. Therefore, the photovoltaic performance of 2D quinoxaline-based D–A-type polymers is significantly affected by the type of electron-withdrawing substituent.

Comparison of TiO2 and ZnO electron selective layers on the inverted-type polymer solar cells

In this study, P3HT:PCBM polymer-based solar cell devices with TiO2 and ZnO electron selective layers were separately investigated to compare effects of metal oxide layer in the inverted-type polymer solar cell structures. R.f. magnetron sputtering technique was used to fabricate highly oriented polycrystalline TiO2 and ZnO thin film layers onto FTO-coated glass substrates. The surface morphology, structural and optical properties of TiO2 and ZnO electron selective layers were studied by performing X-ray diffraction (XRD), confocal Raman spectroscopy, transmittance spectra and Atomic Force Microscope (AFM). Photovoltaic performance of inverted-type polymer solar cell based on Poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-Phenyl C61 butyric acid methyl ester (PCBM) with different electron selective layers was executed to analyze the effects of TiO2 and ZnO thin film layers on the power conversion efficiency (PCE) value of inverted polymer solar cell (IPSC) devices. Produced devices with TiO2 and ZnO thin film indicated the best device performance with maximum PCE values of 2.88 and 3.49%, respectively.

Direct Evidence of Less Charge Accumulation in Highly Durable Polymer Solar Cells Using Operando Electron Spin Resonance Spectroscopy

Inverted-type polymer solar cells with poly[naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole-5,10-diyl[3,4′-bis(2-butyloctyl)[2,2′-bithiophene]-5,5′-diyl]thiazolo[5,4-d]thiazole-2,5-diyl[3′,4-bis(2-butyloctyl)[2,2′-bithiophene]-5,5′-diyl]] (PTzNTz) have attracted attention because of their high power conversion efficiency and thermal stability. However, the internal deterioration mechanism has not yet been completely clarified. Here, we report operand electron spin resonance (ESR) spectroscopy of the highly durable PTzNTz polymer solar cells to investigate the mechanism of the device-performance deterioration at a molecular level. We identify the layer with accumulated charges in the PTzNTz cells by the analyses of light-induced components in the ESR spectra of the thin films and cells with PTzNTz. We observe the correlation between the increase in the number of spins and the decrease in the device performance. Notably, compared to the previously reported ESR studies of polymer solar cells, the PTzNTz solar cel...

Laminated Carbon Nanotubes for the Facile Fabrication of Cost-Effective Polymer Solar Cells

In this study, we fabricate a metal- and vacuum-free electrode composed of aligned multiwall carbon nanotubes (MWCNTs) as top electrode in inverted type polymer solar cells. Simply drawn MWCNT sheets from vertically grown arrays were directly transferred over the top layer (PEDOT:PSS) of the device as an anode material. Without any post treatment, devices exhibited a short-circuit current density (Jsc) of 7.53 mA cm–2, which is 78% of that of devices employing a thermally deposited silver-based anode (Jsc = 9.69 mA cm–2). Scanning electron microscopy images of the device (top and cross-sectional views) show the successful incorporation of MWCNT sheets over the surface of the device without any damage. The thickness of the MWCNT sheets as top electrode is also investigated, and it has been demonstrated that when the sheets’ thickness increased, the conductance of the electrode increased, which showed improvement for the device performance. The attributes of being flexible, vacuum-free, and stable with rega...

Alcohol-soluble conjugated oligomers as the cathode interfacial layer in polymer solar cells

ABSTRACTTwo easily accessible fluorene-based conjugated oligo-electrolytes (COEs) FTF- and FBF-NBr have been developed as the cathode interfacial layer (CIL) in inverted type polymer solar cells (iPSCs). CILsare interpretative to improving the power conversion efficiency (PCE) and long-term stability of the polymer photovoltaic cell that utilizes a high work function cathode. Compared to the reference devices without interlayer, FBF- and FTF-NBr exhibit significant improvements of the device parameters by reducing the work function of indium thin oxide (ITO). Conjugatedoligo-electrolytes in this work havelow lying HOMO levels −5.54 eV for FTF-NBr and −5.77 eV for FBF-NBr which are beneficial to hole-blocking ability. The iPSCs with FBF- and FTF-NBr as the inter layer at the cathode side were fabricated to investigate the effect of CIL on the photovoltaic properties. As a result, the PCE of 7.89% with FBF-NBr and 8.05% with FTF-NBr as the CIL has been achieved. Thus, the results indicate that fluorene-based COEs are potential CIL materials for the iPSCs.

Orienting the Microstructure Evolution of Copper Phthalocyanine as an Anode Interlayer in Inverted Polymer Solar Cells for High Performance.

Recent advances in the interfacial modification of inverted-type polymer solar cells (PSCs) have resulted from controlling the surface energy of the cathode-modified layer (TiO2 or ZnO) to enhance the short-circuit current (Jsc) or optimizing the contact morphology of the cathode (indium tin oxide or fluorine-doped tin oxide) and active layer to increase the fill factor. Herein, we report that the performance enhancement of PSCs is achieved by incorporating a donor macromolecule copper phthalocyanine (CuPc) as an anode modification layer. Using the approach based on orienting the microstructure evolution, uniformly dispersed island-shaped CuPc spot accumulations are built on the top of PTB7:PC71BM blend film, leading to an efficient spectral absorption and photogenerated exciton splitting. The best power conversion efficiency of PSCs is increased up to 9.726%. In addition to the enhanced light absorption, the tailored anode energy level alignment and optimized boundary morphology by incorporating the CuPc interlayer boost charge extraction efficiency and suppress the interfacial molecular recombination. These results demonstrate that surface morphology induction through molecular deposition is an effective method to improve the performance of PSCs, which reveals the potential implications of the interlayer between the organic active layer and the electrode buffer layer.

Cathode modification by the electrostatically self-assembled poly(4-vinylpyridine) derivative for enhancing the performance of polymer solar cells

ABSTRACTA non-conjugated polyelectrolyte, poly(4-vinyl N-ethyl pyridinium bromide) (PVPy-EtBr), is applied to polymer solar cells (PSCs) as a cathode buffer layer either inverted or conventional type polymer solar cells to improve the performances. The Kelvin probe microscopy measurement support the formation of favorable interface dipole at the cathode interface, indicating the reduction of a Schottky barrier for an electron collection from the active layer to the cathode treated with electrostatically self-assembly of PVPy-EtBr. Inverted type PSC with PVPy-EtBr as a cathode buffer layer demonstrate the power conversion efficiency (PCE) of 3.49%, which is better than the device without interlayer (PCE = 2.95%). Most increase in the PCE of the devices with interlayer is resulted from enhancement of the Jsc. This is due to that the reduction of a Schottky barrier at the cathode interface. Conventional type PSC with PVPy-EtBr (3.45%) at the cathode interface also exhibit better the PCE compared to that of the device without interlayer (2.88%).

Step-by-step improvement in photovoltaic properties of fluorinated quinoxaline-based low-band-gap polymers

A series of fluorinated quinoxaline-based polymers with typical donor-π-acceptor configurations has been synthesized by Stille coupling reaction. The electron-donating dialkoxy-substituted benzodithiophene (BDT) is connected to the electron-withdrawing 2,3-diphenylquinoxaline (DPQ) acceptor through a thiophene bridge. To investigate the effect of strong electron-withdrawing moieties, fluorine atoms have been systematically incorporated at various positions on the DPQ units such as the 6,7-positions, the para-positions of the phenyl substituent on the 2,3-positions, and both locations to afford PBDT-QxF, PBDT-FQx, and PBDT-FQxF, respectively. Because of significant contributions from the fluorine atoms, these polymers display quite differentiated optical and electrochemical properties in comparison with those of their non-fluorinated counterpart, PBDT-Qx. In addition, a gradual improvement in power conversion efficiencies (PCEs) is observed in the order of PBDT-Qx, PBDT-QxF, PBDT-FQx, and PBDT-FQxF, when inverted-type polymer solar cells (PSCs) with a configuration of ITO/ZnO/polymers:PC71BM/MoO3/Al were fabricated. In particular, PBDT-FQxF, with four fluorine atoms on the 6,7-positions and the para-positions of the phenyl substituent on 2,3-positions of DPQ, exhibits the highest PCE of 6.60% with an open-circuit voltage (Voc) of 0.91 V, a short-circuit current density (Jsc) of 10.15 mA/cm−2, and a fill factor (FF) of 71.5%.

Direct observation of UV-induced charge accumulation in inverted-type polymer solar cells with a TiOx layer: Microscopic elucidation of the light-soaking phenomenon

The mechanism of light-soaking phenomenon in inverted-type organic solar cells (IOSCs) with a structure of indium-tin-oxide/TiOx/P3HT:PCBM/Au was studied by electron spin resonance (ESR) spectroscopy. Charge accumulation in the cell during UV-light irradiation was observed using ESR, which was clearly correlated with the light-soaking phenomenon. The origin of the charge accumulation is clarified as holes that are deeply trapped at p-type P3HT polymer-chain ends with bromine after hole transfer from the band excitation in the TiOx layer. The holes are considered to be electrostatically attracted to trapped electrons in the TiOx layer after the band excitation. These accumulated charges are the origin of the light-soaking phenomenon. Our results strongly suggest that passivation of the residual OH groups in the TiOx layer is needed to avoid the light-soaking phenomenon by preventing electron trappings, a step that is indispensable in the operation of highly stable IOSCs without UV-light irradiation based on a low-cost and low-temperature device fabrication process using flexible plastic substrates.

Direct observation of UV-induced accumulated charges in inverted-type polymer solar cells with TiOx layer by electron spin resonance

ABSTRACTWe present an electron spin resonance (ESR) study of inverted-type polymer solar cells with the structure of indium tin oxide (ITO)/amorphous titanium oxide (TiOx)/regioregular poly(3-hexylthiophene) (P3HT):phenyl-C61-butryic acid methyl ester (PCBM)/Au. Two light-induced ESR signals with narrow and broad linewidth were observed by light irradiation. We found a strong correlation between typical light-soaking effects and the ESR signal with the broad linewidth under UV light irradiation. From the fact that the light-soaking effect requires UV irradiation, the ESR signal with the broad linewidth is found to be related with the light-soaking effect.

Self‐assembled Poly(4‐vinylpyridine) As an Interfacial Layer for Polymer Solar Cells

A nonconjugated polymer, poly(4‐vinylpyridine) (PVPy), is applied to polymer solar cells (PSCs) as an interfacial layer (IFL) either inverted or conventional type PSCs. The Kelvin probe microscopy measurements support the formation of favorable interface dipole at the cathode interface, indicating the reduction of an electron collection barrier from the active layer to the cathode. Inverted type PSC with PVPy as an IFL demonstrate the power conversion efficiency (PCE) of 3.20%, (open circuit voltage [V oc] = 0.61 V, short circuit current [J sc] = 8.68 mA/cm2, fill factor [FF] = 59.4%), which is better than the device without interlayer (PCE = 2.95%, V oc = 0.60 V, J sc = 8.11 mA/cm2, FF = 60.6%). Most increase in the PCE of the devices with interlayer is resulted from enhancement of the J sc. This is due to that the reduction of an electron collection barrier at the cathode interface. Conventional type PSC with PVPy (3.51%) at the cathode interface also exhibits better the PCE compared to that of the device without interlayer (2.88%).

Mesopore-structured anatase-TiO2 thin films for the electron transport layer in inverted-type polymer solar cells

Mesopore-structured TiO2 thin films for the electron transport layer in inverted-type polymer solar cells have been investigated. The mesopore structure with a diameter of ca. 25 nm was formed on fluorine-doped tin oxide/glass substrates by anodic oxidation. X-ray diffraction measurement reveals that anatase-type TiO2 was formed by the annealing at 500 °C in air. We found that the power conversion efficiency of the inverted-type polymer solar cells was improved by applying a filling process of the photovoltaic materials into the mesopores of TiO2. The power conversion efficiency of 1.58 % was obtained in the solar cell with a filling process for 60 min. The power conversion efficiency of the inverted polymer solar cells with electron transport layer of mesopore-structured TiO2 maintains 97 % of the maximum value even after light irradiation for 300 min.

Cathode modification of polymer solar cells by electrostatically self-assembled zwitterionic non-conjugated polyelectrolyte

A non-conjugated zwitterionic polylectrolyte (PVPy-ZW) based on poly(4-vinylpyridine) is applied to polymer solar cells (PSCs) as a cathode buffer layer either inverted or conventional type polymer solar cells. The Kelvin probe microscopy measurement support the formation of favorable interface dipole at the cathode interface, indicating the reduction of a Schottky barrier for an electron collection from the active layer to the cathode. Inverted type PSC with the PVPy-ZW as a cathode buffer layer demonstrate the power conversion efficiency (PCE) of 3.55% (open circuit voltage (Voc)=0.62V, short circuit current (Jsc)=9.49mA/cm2, fill factor (FF)=60.4%), which is better than the device without interlayer (PCE=2.95%, Voc=0.60V, Jsc=8.11mA/cm2, FF=60.6%). This is due to that the reduction of a Schottky barrier at the cathode interface. Conventional type PSC with PVPy-ZW (3.45%) at the cathode interface also exhibit better the PCE compared to that of the device without interlayer (2.88%). Most increase in the PCE of the devices with interlayer is resulted from enhancement of the Jsc.

High performance inverted polymer solar cells using ultrathin atomic layer deposited TiO2 films

Photovoltaic properties of inverted type polymer solar cells (PSCs) using ultrathin TiO2 layers as an adjuvant electron collecting layer were investigated. To prepare the ultrathin (2.5nm) TiO2 layers on top of TiO2 nanoparticles, atomic layer deposition (ALD) process was conducted at 125, 175, and 200°C. The addition of ALD TiO2 on nanoparticulated TiO2 effectively enhanced the photovoltaic performances of inverted organic solar cells. The inverted PSC device with the thin 200°C-ALD TiO2 layers showed the highest power conversion efficiency of 3.50%, which is an enhancement of approximately 30% compared to the cells without the ALD TiO2 layer (PCE=2.72%). This work demonstrates that the ALD process plays a critical role in the enhancement of electron extraction efficiency with treating the surface defects of the TiO2 nanoparticles.

Effect of self-assembled monolayer treated ZnO as an electron transporting layer on the photovoltaic properties of inverted type polymer solar cells

The work function and the surface property of ZnO can be simply tuned by the self-assembled monolayer (SAM) treatment derived from benzoyl chloride such as 4-fluorobenzoyl chloride (FBC) and 4-tert-butyl benzoyl chloride (BBC). The work function of FBC treated ZnO is 4.75eV, which is larger than that of untreated ZnO (4.55eV). This is due to the formation of unfavorable interface dipole, which is shifted away from the ZnO surface. SAM treated ZnO is used as an electron collection layer for inverted polymer solar cells (iPSCs) with a structure of ITO/ZnO/SAM/active layer (P3HT:PC61BM)/WO3/Ag. Therefore, the open circuit voltage (Voc) of the device based on FBC treated ZnO is 0.53V, which is smaller than that of the device based on bare ZnO (0.58V) and BBC treated ZnO (0.58V). The power conversion efficiency (PCE) of iPSC with FBC treated ZnO is 1.78%, which is smaller than that that of the device with untreated ZnO. The short circuit current (Jsc), the PCE, and the fill factor (FF) of the device with BBC treated ZnO are −9.00mA/cm2, 2.90%, and 55.6%, respectively, which are higher than those of the device with untreated ZnO (−8.63mA/cm2, 2.51%, and 50.2%). Enhancement of the Jsc and FF in the device with BBC treated ZnO are achieved by SAM treatment leading to the reduction of a Schottky barrier at the interface and improvement of morphology of the active layer.

Inverted Type Polymer Solar Cells with Self-Assembled Monolayer Treated ZnO

The work function and surface property of ZnO can be simply tuned by the self-assembled monolayer (SAM) molecules derived from benzoic acid such as 4-methoxybenzoic acid (MBA), 4-tert-butylbenzoic acid (BBA), and 4-fluorobenzoic acid (FBA), which have different dipole orientation and magnitude. MBA, BBA, and FBA treated ZnO layers were used as an electron injection/transporting layer for inverted type polymer solar cells (PSCs) with a structure of ITO/SAM treated ZnO/active layer (P3HT:PC61BM)/MoO3/Ag. The power conversion efficiency (PCE) of PSCs based on MBA and BBA treated ZnO reaches 3.34 and 2.94%, respectively, while the PCE of the device based on untreated ZnO is 2.47%. In contrary, the PCE of the device with FBA treated ZnO is 1.81%. The open circuit voltage (Voc) of the device with MBA, BBA, and FBA treated ZnO is 0.63 and 0.62 V, respectively, while the Voc of PSC with untreated ZnO is 0.60 V. Contrarily, the Voc of the device with FBA treated ZnO is 0.53 V. The PCE and Voc of PSCs based on MBA ...

Efficient light trapping in inverted polymer solar cells by a randomly nanostructured electrode using monodispersed polymer nanoparticles

The randomly nanotextured back electrode provides a simple and efficient route for enhancing photocurrent in polymer solar cells (PSCs) by light trapping, which can increase light absorption within a finite thickness of the active layer. In this study, we incorporated mono-disperse 60 nm polystyrene nanoparticles (PS NPs) into a 50 nm thick poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) anode buffer layer (ABL) to create a randomly nanotextured back electrode with 10 nm height variations in inverted-type PSCs. The roughened interface between the PS NP-PEDOT:PSS ABL and the Ag electrode scatters light in the visible range, leading to efficient light trapping within the device and enhanced light absorption in the active layer. Inverted PSCs with randomly nanotextured electrodes (φ(NP) = 0.31) showed short-circuit current density (J(SC)) and power conversion efficiency (PCE) values that were 15% higher than those of control devices with flat electrodes. External quantum efficiency, reflectance, and optical light scattering as a function of ϕ(NP) were examined to determine the origin of the enhancement in J(SC) and PCE.

Effects of Solubilizing Group Modification in Fullerene Bis-Adducts on Normal and Inverted Type Polymer Solar Cells

Structural control of solubilizing side groups in fullerene-based electron acceptors is critically important to optimize their performance in bulk heterojunction (BHJ)-type polymer solar cell (PSC) devices. The structural changes of fullerene derivatives affect not only their optical and electrochemical properties but also their solubility and miscibility with electron donor polymers. Herein, we synthesized a series of o-xylenyl C60 bis-adduct (OXCBA) derivatives with different solubilizing side groups to systematically investigate the effects of fullerene derivative structures on the photovoltaic properties of PSCs. The xylenyl side groups on the OXCBA were modified to produce several different OXCBA derivatives in which the xylenyl groups were functionalized with fluorine (FXCBA), nitro (NXCBA), methoxy and bromine (BMXCBA), and phenyl groups (ACBA). End group modifications of OXCBA dramatically affect photovoltaic performance in blend films with poly(3-hexylthiophene) (P3HT), resulting in power convers...