Voltage Of Polymer Solar Cells Research Articles

New Donor–Acceptor Random Terpolymers with Wide Absorption Spectra of 300–1000 nm for Photovoltaic Applications

The development of conjugated polymers with wide absorption spectra is imperative to achieve high efficiency in polymer solar cells (PSCs), since most of these polymers usually absorb only a limited range of the solar spectrum. Random terpolymers consisting of three blocks (one electron-donor and two electron-acceptor blocks) are promising p-type polymers for PSCs, because the inclusion of a third block in the polymer macromolecules provides a synergistic effect of physical properties, such as absorption capacity, charge transfer, HOMO/LUMO energy levels, and photovoltaic characteristics. In this regard, we have developed and synthesized random terpolymers consisting of two different chromophores (DPP and BFCTP) with complementary absorption spectra as co-acceptor blocks in conjugated donor–acceptor (D–A) copolymers. Random copolymers exhibit both broad absorption and low HOMO levels favoring short-circuit current and idle voltage in PSCs. It is expected that new terpolymers consisting of one electron-donor unit and two electron-acceptor moieties will make a significant contribution to the development of highly efficient PSCs. It is expected that new ternary copolymers consisting of one electron-donor unit and two electron-acceptor fragments will make a significant contribution to the development of high-performance PSСs.

Synthesis, Optical and Electrochemical Properties of Two Series Electron-Donor Polymers with Diphenylpyranylidene Side Chain

Two series of novel conjugated polymers with diphenylpyranylidene as the side chain and thiophene as the main chain were designed, synthesized and characterized as electron donors of polymer solar cells for the first time. The absorption spectra showed that introduction of diphenylpyranylidene side chain can obviously complement the light absorption ability in the range from 300-500 nm. In addition, diphenylpyranylidene unit is also helpful to tune the HOMO energy level as low as -5.23 to -5.31 eV than P3HT polymers, for favoring higher open-circuit voltage in polymer solar cells.

Manipulating electronic energy levels of wide-bandgap D–A copolymers via side-chain engineering to realize high open-circuit voltage polymer solar cells

Besides high short-circuit current density (Jsc) and fill factor (FF), high open-circuit voltage (Voc) is urgently necessary for obtaining high overall efficiencies of polymer solar cells (PSCs). In order to produce high Voc PSCs, herein, we developed three wide-bandgap donor–acceptor (D-A) alternate copolymers (PBDTO-TPTI, PBDTT-TPTI, and PBDTS-TPTI) of benzodithiophene (BDT) and thienopyridothieno-isoquinoline-5,11(4H,10H)-dione (TPTI) moieties. These copolymers possess a uniform BDT-TPTI framework, but various side chains (alkoxyl, alkylthienyl, alkylthiothienyl) on the BDT unit. The resultant data convincingly reveal that the spectral absorption, optical bandgap (Egopt), aggregation characteristic, energy levels, charge transport properties and active layer morphology of the D–A copolymers can be effectively manipulated via side-chain engineering on the BDT segment. The gradually increased Egopt (1.92–1.95 and then to 1.97 eV) and gradually decreased HOMO/LUMO levels (−5.43/−3.47 to −5.54/−3.53 and then to −5.56/−3.76 eV) are found while the side group on the BDT unit is varied from alkoxyl (PBDTO-TPTI) to alkylthieyl (PBDTT-TPTI) and then to alkylthiothienyl (PBDTS-TPTI). Importantly, the geometric and optoelectronic properties of these polymers are supported by theoretical predictions. PSCs based on all the three copolymers with a fullerene-based acceptor (PC71BM) exhibit power conversion efficiencies (PCEs) exceeding 5% and a Voc over 0.93 V. Notably, PBDTS-TPTI-based PSC achieves the highest PCE of 5.35% accompanied with the highest Voc as far as 0.99 V and Jsc up to 12.60 mA cm−2. This work indicates side-chain engineering on polymers is an impactful and feasible approach to realize high Voc PSCs by manipulating electronic levels of D–A copolymers.

Construction of Interface Dipoles by Surface Doping and Their Role in the Open Circuit Voltage in Polymer Solar Cells

A kind of dipolar interface is realized by surface doping of poly-(3,4-ethylenedioxythiophene) (PEDOT) with tetrafluoro-tetracyano-quinodimethane (F4TCNQ). PEDOT is in situ synthesized by electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) on an indium tin oxide (ITO) electrode, and then F4TCNQ is spin-coated atop the PEDOT layer. Because the LUMO of F4TCNQ is lower than the HOMO of PEDOT, the spontaneous electron transfer from PEDOT to F4TCNQ results in a bilayered structure of PEDOT cations and F4TCNQ anions. Thus, a permanent interfacial dipole is formed in the surface-doping system. The surface doping not only enhances the conductivity of PEDOT, but also increases the surface work function of the electrode. The dipolar film is applied as the anode interface in polymer solar cells (PSCs), and the results show that such an interface dipole plays a very important role in the open circuit voltage (V oc) of the PSCs.

Impact of charge carrier recombination and energy disorder on the open-circuit voltage of polymer solar cells

Charge carrier recombination and energy disorder in organic solar cells both have a profound impact on the open-circuit voltage of the device. In this paper, both traditional fullerene-(PC<sub>71</sub>BM) and nonfullerene-(O-IDTBR) based solar cells were fabricated using the same electron donor material (PTB7-Th). The room-temperature current density–voltage characteristics showed that despite the values of their power conversion efficiencies were very close, there was a huge open circuit voltage (<i>V</i><sub>oc</sub>) difference between the PC<sub>71</sub>BM and O-IDTBR devices. To understand the sources of the <i>V</i><sub>oc</sub> variation, characterization techniques such as impedance spectra, low temperature electrical characterization method, transient photovoltage, and electroluminescent spectra were carried out. Temperature-dependent <i>V</i><sub>oc</sub> of the devices were measured in a large temperature range between 120 K and 300 K. The charge transfer state energy (<i>E</i><sub>CT</sub><italic/>) of the fullerene and the nonfullerene cells were determined to be 1.13 V and 1.34 V, respectively. Furthermore, the Mott-Schottky equation was applied to analyze the capacitance- voltage curves of the fabricated devices. Results showed that the built-in voltage (<i>V</i><sub>bi</sub>) of the O-IDTBR based cell (1.38 V) was much higher than that of the PC<sub>71</sub>BM cell (1.15 V). By analyzing the above data, it was easy to speculate that charge carrier recombination loss in the PC<sub>71</sub>BM device was more serious since the net electric field was relatively weak. Impedance spectra were used to measure the charge carrier recombination process in both devices. Fitting results through the equivalent circuit stated clearly that values of the recombination resistance in the O-IDTBR device were much higher in the test range, indicating that the charge carrier was less easy to recombine in the nonfullerene device. This speculation could be verified by the transient photovoltage (TPV) measurements since the carrier lifetime in the O-IDTBR device was much longer. The excited states in the devices were investigated by the electroluminescence spectra. Since the full width at half maximum (FWHM) of the O-IDTBR emission spectrum was narrower, the excited state energy distribution in the O-IDTBR device was more uniform. Based on the above analyses, the higher <i>V</i><sub>oc</sub> in the O-IDTBR device was attributed to the mild charge carrier recombination and low energy disorder.

4-Methylthio substitution on benzodithiophene-based conjugated polymers for high open-circuit voltage polymer solar cells

Abstract Two new two-dimensional conjugated polymers PBDTT-4S-TT and PBDTT-4S-BDD based on benzo[1,2-b:4,5-b']dithiophene unit with 4-methylthio substituted thiophene side chains were designed and synthesized. The two polymers exhibit slightly blue-shifted absorptions and deeper-lying highest occupied molecular orbital energy level compared with their analogues without 4-methylthio substituents. Polymer solar cells of the two polymers employing 6,6-phenyl-C71-butyric acid methyl ester as the electron acceptor afford an open-circuit voltage of up to 0.98 V, which is much higher than that of their parent polymers without 4-methylthio substituents. The highest power conversion efficiencies of 6.6% and 7.8% were realized by PBDTT-4S-TT and PBDTT-4S-BDD, respectively. These results suggest that 4-methylthio substitution on thiophene side chain of two-dimensional polymers is an effective strategy to enhance the Voc of polymer solar cells.

Using ternary blend as a strategy to improve the driving force for charge transfer and facilitate electron transport in polymer solar cells

Abstract The recently emerged non-fullerene small molecular acceptors can enhance the light absorption and increase the open-circuit voltage in polymer solar cells (PSCs); however, sometimes a decrease in the driving force for charge transfer from donor to acceptor is accompanied. Here, we reported a ternary blend strategy by utilizing fullerene derivative (PC71BM) as a third component to improve the driving force for charge transfer, the charge transport, and the film morphology. Accordingly, the ternary blend PSCs demonstrate a much enhanced power conversion efficiency (PCE) with an optimized value of 9.18% and a good fill factor (FF) of 63%. In comparison, the non-fullerene PSCs without the third component only show a PCE of 6.90% and an FF value of 56%.

Silaindacenodithiophene‐Based Fused‐Ring Non‐Fullerene Electron Acceptor for Efficient Polymer Solar Cells

AbstractIn this work, a new A‐D‐A type nonfullerene small molecular acceptor SiIDT‐IC, with a fused‐ring silaindacenodithiophene (SiIDT) as D unit and 2‐(3‐oxo‐2,3‐dihydroinden‐1‐ylidene)malononitrile (INCN) as the end A unit, was design and synthesized. The SiIDT‐IC film shows absorption peak and edge at 695 and 733 nm, respectively. The HOMO and LUMO of SiIDT‐IC are of −5.47 and −3.78 eV, respectively. Compared with carbon‐bridging, the Si‐bridging can result in an upper‐lying LUMO level of an acceptor, which is benefit to achieve a higher open‐circuit voltage in polymer solar cells (PSCs). Complementary absorption and suitable energy level alignment between SiIDT‐IC and wide bandgap polymer donor PBDB‐T were found. For the PBDB‐T:SiIDT‐IC based inverted PSCs, a D/A ratio of 1: 1 was optimal to achieve a power conversion efficiency (PCE) of 7.27%. With thermal annealing (TA) of the blend film, a higher PCE of 8.16% could be realized due to increasing of both short‐circuit current density and fill factor. After the TA treatment, hole and electron mobilities were elevated to 3.42 × 10−4 and 1.02 × 10−4 cm2·V−1·s−1, respectively. The results suggest that the SiIDT, a Si‐bridged fused ring, is a valuable D unit to construct efficient nonfullerene acceptors for PSCs.

Molecular design of novel fullerene-based acceptors for enhancing the open circuit voltage in polymer solar cells

Organic solar cells, especially bulk hetero-junction polymer solar cells (PSCs), are the most successful structures for applications in renewable energy. The dramatic improvement in the performance of PSCs has increased demand for new conjugated polymer donors and fullerene derivative acceptors. In the present study, quantum chemical calculations were performed for several representative fullerene derivatives in order to determine their frontier orbital energy levels and electronic structures, thereby helping to enhance their performance in PSC devices. We found correlations between the theoretical lowest unoccupied molecular orbital levels and electrophilicity index of various fullerenes with the experimental open circuit voltage of photovoltaic devices according to the poly(3-hexylthiophene) (P3HT):fullerene blend. The correlations between the structure and descriptors may facilitate screening of the best fullerene acceptor for the P3HT donor. Thus, we considered fullerenes with new functional groups and we predicted the output factors for the corresponding P3HT:fullerene blend devices. The results showed that fullerene derivatives based on thieno-o-quinodimethane-C60 with a methoxy group will have enhanced photovoltaic properties. Our results may facilitate the design of new fullerenes and the development of favorable acceptors for use in photovoltaic applications.

An approach to high open-circuit voltage polymer solar cells via alcohol/water-soluble cathode interlayers based on anthrathiadiazole derivatives

Two water/alcohol-soluble small molecular cathode interlayers (CILs) were synthesized and employed to fabricate high performance polymer solar cells (PSCs) with a large open-circuit voltage (Voc) of 0.93 V.

Tuning the LUMO level of organic photovoltaic solar cells by conjugately fusing graphene flake: A DFT-B3LYP study

The efficiency of polymer solar cells can be essentially enhanced by improving the performance of electron acceptor materials especially by increasing its lowest unoccupied molecular orbital (LUMO) level. To this aim, the electronic properties of an extremely π-extended system, graphene flake and functionalized graphene flake with phenyl-C-butyric methyl ester (PCBM) group, were investigated and compared with those of C60 ones. Several properties of phenyl-C61-butyric methyl ester ([C60]PCBM) derivatives have been calculated and compared against the experimental and other theoretical results. All geometrical and electronic structures calculations were carried out by using the density functional theory (DFT) method at the B3LYP level of theory. The results show that the phenyl ring of [C60]PCBM was substituted with the methoxy groups to increase the LUMO level of the respective system which is in agreement with other studies. We found that graphene flake functionalized by methoxy-PCBM group offer significant increase in the value of the LUMO energy level in comparison with C60 counterpart. Furthermore, the electrophilicity of this compound is lower than that of the other counterparts, which results in higher open circuit voltage (Voc) value in the photovoltaic device. These findings could provide fundamental insights in improving the Voc value as well as raising the LUMO levels of electron acceptor materials and may also serve as an alternative strategy to increase open circuit voltage in polymer solar cells.

An effective way to reduce energy loss and enhance open-circuit voltage in polymer solar cells based on a diketopyrrolopyrrole polymer containing three regular alternating units

A planar DPP-based polymer containing three regular alternating units exhibits a good photovoltaic performance with a high PCE of 9.02% and a large Voc of 0.86 V.

Incorporating a vertical BDT unit in conjugated polymers for drastically improving the open-circuit voltage of polymer solar cells

High open circuit voltages (Vocs) of over 1.0 V were obtained for solar cell devices based on vertical BDT-containing conjugated polymers.

High open circuit voltage polymer solar cells enabled by employing thiazoles in semiconducting polymers

Introduction of thiazole into the conjugated backbone of semiconducting polymers is a useful strategy to enhance the open-circuit voltage of polymer solar cells.

Benzocyclobutene-fullerene bisadducts as novel electron acceptors for enhancing open-circuit voltage in polymer solar cells

A novel 56 π-electron fullerene acceptor material, benzocyclobutene-C60 bisadducts (BCBCBA), was developed via the [2+2] cycloaddition of in-situ generated benzyne with C60 to raise the lowest unoccupied molecular orbital (LUMO) energy level of the fullerene. This four-membered linkages to C60 of BCBCBA resulted in the fullerene bisadducts with very addends, thus minimizing the adverse effects of their regioisomers on the intermolecular packing between fullerene bisadducts. In addition, the LUMO level of BCBCBA was as high as −3.70eV, which is 0.16eV higher than that of phenyl-C61-butyric acid methyl ester (PCBM) (−3.86eV). This result indicates the significant potential for improving open-circuit voltage (VOC) values in polymer solar cells (PSCs). The poly(3-hexylthiophene) (P3HT):BCBCBA based PSCs show VOC of 0.73 and power conversion efficiency (PCE) of 3.51%, which are similar to the P3HT:bis-PCBM based PSCs.

Influence of the interface material layers and semiconductor energetic disorder on the open circuit voltage in polymer solar cells

ABSTRACTMaterial layers at electrode/semiconductor interfaces are fundamental for the photovoltaic properties of polymer solar cells. The relationship between open‐circuit voltage (VOC) and the work function (φ) of these interface layers is still a matter of debate. Simulations, together with experiments on over more than 20 cell architectures based on P3HT:PC60BM, enabled us to analyze the physical dependence of VOC on φ. In particular, when the work function of the contacts is well inside the gap we observe that performance depends strongly on even small variations of φ. On the other hand, when it approaches the energy levels of the semiconducting polymers, device operation becomes the most efficient and less sensitive to variations in φ. Furthermore, by varying the Gaussian density of states (DOS) of the active blend we were able to show that VOC performance depends significantly also on the DOS. Our study based on the simultaneous variation of transport layers represents a promising method to optimize the design and performance of polymer solar cells. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 690–699

Effective side chain selection for enhanced open circuit voltage of polymer solar cells based on 2D-conjugated anthracene derivatives

Two novel conjugated donor–acceptor polymers were synthesized by combining anthracene donor and benzothiadiazole acceptor with two different side chains (conjugated and non-conjugated). Their absorption spectroscopy, thermogravimetric analysis, electrochemical cyclic voltammetry, theoretical prediction, surface morphology, and photovoltaic performance were investigated. The resultant two-dimensional configuration showed good optical and electrochemical properties. By selectively introducing branched conjugated side chains and non-conjugated linear side chains on the polymer back-bone, the highest occupied molecular orbital levels are low lying which results in an increased open circuit voltage for polymer solar cells. The open circuit voltage of 1.02 V in this work was among the highest value for anthracene-based polymer solar cells ever. Our results suggest a good way to regulate the molecular energy levels by selecting appropriate side chains.

Effect of doping on the short-circuit current and open-circuit voltage of polymer solar cells

The change in doping density in P3HT:PCBM based polymer solar cells (PSCs) with different processing solvents and with/without post-fabrication thermal treatment is investigated with capacitance-voltage measurement and optical microscopic imaging. The results suggest that both slow drying and thermal treatment facilitate the phase-separation and crystallinity of P3HT and PCBM, leading to low defect density and thus low p-type doping. Direct links between the doping density and the performance of the PSCs, specifically the short-circuit current (Jsc) and open-circuit voltage (Voc), are observed. The results show that doping density is one of the decisive factors affecting the photocurrent of the PSCs. Lower doping density leads to a wider depletion region, which is beneficial for carrier collection. The agreement between the calculation and the experiment suggests that the Voc increases monotonically with increasing doping densities in the PSCs. These rules consistently explain our results on the change of Jsc and Voc after thermal annealing in the PSCs with different processing solvents.

Finding the lost open-circuit voltage in polymer solar cells by UV-ozone treatment of the nickel acetate anode buffer layer.

Efficient polymer solar cells (PSCs) with enhanced open-circuit voltage (Voc) are fabricated by introducing solution-processed and UV-ozone (UVO)-treated nickel acetate (O-NiAc) as an anode buffer layer. According to X-ray photoelectron spectroscopy data, NiAc partially decomposed to NiOOH during the UVO treatment. NiOOH is a dipole species, which leads to an increase in the work function (as confirmed by ultraviolet photoemission spectroscopy), thus benefitting the formation of ohmic contact between the anode and photoactive layer and leading to increased Voc. In addition, the UVO treatment improves the wettability between the substrate and solvent of the active layer, which facilitates the formation of an upper photoactive layer with better morphology. Further, the O-NiAc layer can decrease the series resistance (Rs) and increase the parallel resistance (Rp) of the devices, inducing enhanced Voc in comparison with the as-prepared NiAc-buffered control devices without UVO treatment. For PSCs based on the P3HT:PCBM system, Voc increases from 0.50 to 0.60 V after the NiAc buffer layer undergoes UVO treatment. Similarly, in the P3HT:ICBA system, the Voc value of the device with a UVO-treated NiAc buffer layer increases from 0.78 to 0.88 V, showing an enhanced power conversion efficiency of 6.64%.

Enhanced open-circuit voltage in polymer solar cells by dithieno[3,2-b:2′,3′-d]pyrrole N-acylation

N-Acylation of dithieno[3,2-b:2′,3′-d]pyrrole (DTP) leads to enhanced open-circuit voltages and hence higher power conversion efficiencies in polymer solar cells.