PCBM Solar Cells Research Articles

Investigation on the Effects of Additive on Performance and Photostability of PCDTBT:PC71BM Solar Cells

In this work, the effect of DIM on the PCE and photostability of PCDTBT:PC71BM PSCs was investigated. DIM is an effective additive in a BHJ PCDTBT:PC71BM solar cell since it fulfills the requirement of a selective PC71BM dissolution. PCE of the device based on PCDTBT:PC71BM processed with DIM is higher than that of the reference device. In terms of the device stability, the PSCs processed with DIM showed poor stability at longer light exposure time. For the device without DIM especially as the light exposure time was increased, the device stability was better because the PCDTBT could be shielded from air by an aggregated PC71BM layer. For the PCDTBT:PC71BM device processed with DIM, the results obtained from J‐V measurement indicates that it has a lower recombination rate. The result from IS measurement shows that for pristine PCDTBT:PC71BM devices with 3% DIM, the active layer resistance is lower compared to the device without DIM. However, after irradiating the device for 5 hr, the resistance of the device processed with DIM is higher and it is consistent with decreased PCE of the aged device.

Enhancement of the Power Conversion Efficiency of Organic Solar Cells by Surface Patterning of Azobenzene Thin Films.

Nanoscale-patterned azobenzene thin films were incorporated in organic solar cells to scatter incident light, thus increasing the optical path length of photons inside the active area. This ultimately led to significant power conversion efficiency (PCE) enhancements in the active layer. Specifically, the azobenzene thin films were patterned with two-dimensional crossed surface relief gratings inscribed via laser interference lithography. The patterned films were then bleached and thermally stabilized by exposure to strong ultraviolet light before being incorporated in P3HT:PC61BM and PTB7:PC61BM solar cells. The fabricated solar cells exhibited a PCE enhancement of 133%, from 1.37 to 3.19%, for P3HT:PC61BM solar cells, and a PCE enhancement of 302%, from 0.53 to 2.13%, for PTB7:PC61BM solar cells.

Nearly 100% Photocrosslinking Efficiency in Ultrahigh Work Function Hole-Doped Conjugated Polymers Using Bis(fluorophenyl azide) Additives.

Self-compensated (SC) hole-doped conjugated polyelectrolytes with high work functions can provide efficient hole-injection and -collection layers for organic and other semiconductor devices. If these films can be photocrosslinked, the semiconductor overlayer can be deposited from a wider range of solvents, enabling flexibility in device design and fabrication. However, a generic photocrosslinking methodology for these materials is not yet available. Here, we demonstrate that sFPA82-TfO, the recently developed bis(fluorophenyl azide) photocrosslinker that is also i-line compatible, can surprisingly give 100% efficient photocrosslinking for SC hole-doped conjugated polyelectrolytes, i.e., one crosslink per reactive moiety, using mTFF-C2F5SIS-Na, a triarylamine-fluorene copolymer, as the model polyelectrolyte, without degrading its ultrahigh work function of 5.75 eV. The photocrosslinking efficiency is much higher than in the corresponding undoped polyelectrolyte and nonconjugated polyelectrolyte films, where the efficiency is only 20%. We attribute this improvement to the formation of smaller ion multiplet clusters in the hole-doped polyelectrolyte, as suggested by molecular dynamics simulations and infrared spectroscopy, which prevents occlusion of the ionic crosslinker. Photocrosslinking of the SC hole-doped mTFF-C2F5SIS-Na film used as a hole-injection layer in 100 nm-thick PFOP diodes suppresses the leakage current by over 3 orders of magnitude compared to those without crosslinking, to below 30 nA cm-2 at ±2 V. Photocrosslinking of the same film used as the hole-collection layer in PBDTTPD:PC61BM solar cells produces a higher photocurrent density, fill factor, and power conversion efficiency.

Networked Conductive Polythiophene/Polyaniline Bottlebrushes with Modified Carbon Nanotubes As Hole Transport Layer in Organic Photovoltaics

Three types of hole transport layers (HTLs) were developed based on multi-walled carbon nanotubes (CNTs), including pure CNT thin films, CNT:poly(3-thiophene ethanol) (P3ThEt)-g-polyaniline (PANI) nanocomposites, and CNT-g-poly(3-dodecyl thiophene) (PDDT):P3ThEt-g-PANI interconnected networks, and utilized in poly[benzodithiophene-bis(decyltetradecyl-thien) naphthothiadiazole] (PBDT-DTNT):phenyl-C61-butyric acid methyl ester (PC61BM) and poly[bis(triiso-propylsilylethynyl) benzodithiophene-bis(decyltetradecyl-thien) naphthobisthiadiazole] (PBDT-TIPS-DTNT-DT):PC61BM solar cells. Pure CNTs were not the appropriate candidates for application instead of conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTLs. To tackle this issue, the CNT:P3ThEt-g-PANI and CNT-g-PDDT:P3ThEt-g-PANI films were focused with thicknesses of 10 nm and 20 nm. The prominent characteristics peaked at 20-nm thin films of CNT-g-PDDT:P3ThEt-g-PANI, demonstrating the largest power conversion efficiencies (PCE) of 5.65 (12.84 mA/cm2, 62%, and 0.71 V) and 4.80% (11.59 mA/cm2, 60%, and 0.69 V) in the BDT-DTNT and PBDT-TIPS-DTNT-DT based devices, respectively. The CNT-g-PDDT:P3ThEt-g-PANI thin films which possess an interconnected network, composed of grafted-CNTs and P3ThEt-g-PANI bottlebrushes, were proper alternatives for conventional PEDOT:PSS HTLs and warranted the superior photovoltaic results by smooth morphologies (root mean square = 1.0–1.1 nm) and low sheet resistance (2.2–8.3 × 104 Ω/sq). The corresponding systems without grafting of CNT precursors were the second categories of well-functioned HTLs (3.13–4.04%) and had somehow decreased physical and photovoltaic properties.

RETRACTED ARTICLE: Electrode buffer layers via networks of polythiophene/polyaniline bottlebrushes and carbon nanotubes in organic solar cells

A new p-type electrode buffer layer (EBL) material was developed by the networks composed of multi-walled CNTs (MWCNTs) and poly(3-thiophene ethanol) (P3ThEt)-g-polyaniline (PANI) bottlebrush copolymers. The nanocomposites of CNT:P3ThEt-g-PANI were prepared in three different thicknesses (5, 15, and 25 nm) and employed as hole transport layer (HTL) in poly(3-hexylthiophene) (P3HT):phenyl-C71-butyric acid methyl ester (PC71BM) solar cells. A trade-off was detected between the sheet resistance and transmittance by elevating the HTL thickness for both pure CNT and CNT:P3ThEt-g-PANI nanocomposites. The CNT:P3ThEt-g-PANI thin films, in particular with an optimal thickness of 15 nm, were the turning points for equilibrating the film thickness, transmittance, surface roughness, and sheet resistance values. The smoothest thin films of CNT:P3ThEt-g-PANI with the thickness of 15 nm, the transmittance of 85–89%, and the sheet resistance of 5.6 × 104 Ω/sq reflected the best results of 12.85 mA/cm2, 60.7%, and 0.68 V. Hence, a maximum power conversion efficiency (PCE) of 5.30% was acquired among all solar cells fabricated in current work. After peaking at 15 nm, the second group of proper results was recognized in CNT:P3ThEt-g-PANI (25 nm)/P3HT:PC71BM photovoltaics (10.37 mA/cm2, 49.0%, and 0.62 V). The PCE of 3.15% for this system was even greater than the ideal performance (= 2.94%) detected in the pure CNT (15 nm)/P3HT:PC71BM solar cells.

Combined parametric optimization of P3HT: PC70BM films for efficient bulk-heterojunction solar cells

In this report, the effects of photoactive blend compositions, film thicknesses, and annealing conditions on the P3HT:PC70BM solar cells performance and reproducibility was investigated. The performance of prepared devices was described by examining their absorption spectra, current-voltage characteristics and external quantum efficiency (EQE). The thickness of active layer was achieved as 190 nm, 125 nm, and 90 nm, by maintaining the spin speed. Current density (Jsc) slightly increases from 6.39 to 7.15 mA/cm2 with increase in thickness from 90 to 125 nm; however, with further increase in film thickness (190 nm), the Jsc was reduced to 4.39 mA/cm2. To optimize the device performance, four different compositions of PC70BM (1:0.6, 1:0.8, 1:1, and 1:12) were investigated at the most favorable film thickness ~ 125 nm. The effect of different PC70BM compositions on photovoltaic performance was demonstrated by X-ray diffraction (XRD) and Raman measurements that illuminated modification in structural properties. Additionally, annealing condition led to achieve the good phase separation for efficient charge separation and transport within P3HT: PCBM film which further leads to increased efficiency (PCE ~ 3.31%). These effects deliver valued facts for the choices of PC70BM amount in P3HT:PC70BM system, and this efficient device optimization might be useful in other efficient photovoltaic systems for better performance through excellent reproducibility.

Core-Double Shell Nano-hybrids Designed by Multi-walled Carbon Nanotubes, Polyaniline and Polythiophenes in PBDT-DTNT:PC61BM Solar Cells

Core–mantle–shell supramolecules composed of carbon nanotube (CNT)-graft-polyaniline (PANI), poly(3-hexylthiophene) (P3HT), and poly[benzodithiophene-bis(decyltetradecyl-thien) naphthothiadiazole] (PBDT-DTNT) precursors were designed and utilized in PBDT-DTNT:phenyl-C61-butyric acid methyl ester (PC61BM) solar cells. Weight ratio of polymer:CNT-graft-PANI was 9:1 and the weight ratios were 1:1 in binary and 1:1:1 in ternary systems. Diameters of core(CNT)–mantle(PANI), core(CNT)–mantle(PANI)–shell(P3HT), and core(CNT)–mantle(PANI)–shell(PBDT-DTNT) nanostructures ranged in 75–90 nm, 145–160 nm, and 120–130 nm, respectively. Efficacies of 6.82% (13.92 mA/cm2, 0.71 V, 69%, 7.1 × 10−3 cm2/V s and 1.9 × 10−2 cm2/V s) and 7.60% (14.66 mA/cm2, 0.73 V, 71%, 9.0 × 10−3 cm2/V s and 3.4 × 10−2 cm2/V s) were acquired for photovoltaics based on the nanostructures having PBDT-DTNT and P3HT shells, respectively. The PANI mantle may act as both acceptor (accepting the electrons from core) and donor (donating the electrons to shell) in the configuration of core–mantle–shell supramolecules. The P3HT shells acted better than the PBDT-DTNT ones, originated from the simple structure of P3HT backbones and their more ordered and thicker shells, and thus had larger charge mobilities and currents.

Effect of polydispersity on the bulk‐heterojunction morphology of P3HT:PCBM solar cells

ABSTRACTBulk heterojunctions (BHJs) based on semiconducting electron–donor polymer and electron–acceptor fullerene have been extensively investigated as potential photoactive layers for organic solar cells (OSCs). In the experimental studies, poly‐(3‐hexyl‐thiophene) (P3HT) polymers are hardly monodisperse as the synthesis of highly monodisperse polymer mixture is a near impossible task to achieve. However, the majority of the computational efforts on P3HT: phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM)‐based OSCs, a monodisperse P3HT is usually considered. Here, results from coarse‐grained molecular dynamics simulations of solvent evaporation and thermal annealing process of the BHJ are shared describing the effect of variability in molecular weight (also known as polydispersity) on the morphology of the active layer. Results affirm that polydispersity is beneficial for charge separation as the interfacial area is observed to increase with higher dispersity. Calculations of percolation and orientation tensors, on the other hand, reveal that a certain polydispersity index ranging between 1.05 and 1.10 should be maintained for optimal charge transport. Most importantly, these results point out that the consideration of polydispersity should be considered in computational studies of polymer‐based OSCs. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 895–903

Residual solvent additive enables the nanostructuring of PTB7-Th:PC71BM solar cells via soft lithography

The nanoimprinting of polymer solar cells via soft lithography is an attractive approach for enhancing light absorption in the active layer. Many efficient polymer solar cells utilize a high boiling point solvent additive such as 1,8-diiodooctane (DIO) during active layer deposition to optimize morphology. By studying active layer films based on the PTB7-Th:PC71BM system prepared with different amounts of the solvent additive DIO, it is shown that the soft imprinting of such blends critically relies upon the presence of residual solvent additive that plasticizes the film. In particular, a minimum of ∼ 2 volume % of DIO in the casting solution is found to be necessary to enable effective imprinting. The microstructure of imprinted layers is also comprehensively characterized using atomic force microscopy, grazing incidence wide-angle X-ray scattering and resonant soft X-ray scattering, demonstrating that soft lithography can be used to effectively impart structure on the a photonic length scale without changing the nanoscale morphology and microstructure.

Comprehensive Investigation and Analysis of Bulk-Heterojunction Microstructure of High-Performance PCE11:PCBM Solar Cells

Worldwide research efforts have been devoted to organic photovoltaics in the hope of a large-scale commercial application in the near future. To meet the industrial production requirements, organic photovoltaics that can reach power conversion efficiency (PCE) of over 10% along with promising operational device stability are of utmost interest. In the study, we take PCE11:PCBM as a model system, which can achieve over 11% PCE when processed from nonhalogen solvents, to deeply investigate the morphology-performance-stability correlation. We demonstrate that four batches of PCE11 with varying crystalline properties can achieve similar high performance in combination with PCBM. Careful device optimization is necessary in each case to properly address the requirements for the quite distinct microstructures. The bulk-heterojunction (BHJ) microstructure is comprehensively investigated as a function of the macromolecular weight and crystallinity. It is demonstrated that small differences in morphology significantly affect the kinetics and thermodynamic equilibrium of the BHJ microstructure as well as the photostability and thermal stability of the PCE11:PCBM solar cells.

Initial photo-degradation of PCDTBT:PC70BM solar cells studied under various illumination conditions: Role of the hole transport layer

Encapsulated organic solar cells often show a burn-in behaviour under illumination. This burn-in manifests itself as a rapid performance loss followed by a much slower progression of the degradation. Here we investigate the burn-in for PCDTBT:PC70BM solar cells under a wide range of illumination intensities. We find that increasing the sunlight concentration from 1 Sun to up to 100 Suns does not change the degradation behaviour, i.e. the dependence of all principal photovoltaic parameters on the dose of solar exposure (in Sun hours). This suggests that the degradation mechanisms under solar concentration (≤100 Suns) are the same as those observed under 1 Sun. This result makes it possible to use concentrated sunlight for accelerated stability assessment of these devices. We also find that devices with PEDOT:PSS as hole transport material show a rapid drop in open-circuit voltage of around 100 mV during the first Sun hour of light exposure. By replacing PEDOT:PSS with MoO3 this initial process can be prevented and only the much slower part of the photo-degradation takes place.

Polymer/Fullerene Blend Solar Cells with Cadmium Sulfide Thin Film as an Alternative Hole-Blocking Layer.

In this work, chemical bath-deposited cadmium sulfide (CdS) thin films were employed as an alternative hole-blocking layer for inverted poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. CdS films were deposited by chemical bath deposition and their thicknesses were successfully controlled by tailoring the deposition time. The influence of the CdS layer thickness on the performance of P3HT:PCBM solar cells was systematically studied. The short circuit current densities and power conversion efficiencies of P3HT:PCBM solar cells strongly increased until the thickness of the CdS layer was increased to ~70 nm. This was attributed to the suppression of the interfacial charge recombination by the CdS layer, which is consistent with the lower dark current found with the increased CdS layer thickness. A further increase of the CdS layer thickness resulted in a lower short circuit current density due to strong absorption of the CdS layer as evidenced by UV-Vis optical studies. Both the fill factor and open circuit voltage of the solar cells with a CdS layer thickness less than ~50 nm were comparatively lower, and this could be attributed to the effect of pin holes in the CdS film, which reduces the series resistance and increases the charge recombination. Under AM 1.5 illumination (100 mW/cm2) conditions, the optimized PCBM:P3HT solar cells with a chemical bath deposited a CdS layer of thickness 70 nm and showed 50% power conversion efficiency enhancement, in comparison with similar solar cells with optimized dense TiO2 of 50 nm thickness prepared by spray pyrolysis.

Saturation/unsaturation of P3HT:PC71BM photovoltaics with CNT derivatives and correlated supramolecules

Abstract Three different types of multi-walled carbon nanotube (CNT) derivatives such as unmodified CNT, grafted-CNT (CNT-graft-poly(3-hexylthiophene) (P3HT)), and pre-designed supramolecular nanostructure (CNT-graft-poly(3-dodecylthiophene) (PDDT)/P3HT) with weight percentages of 0.0005–50 wt% were applied to be incorporated as third components in P3HT:phenyl-C71-butyric acid methyl ester (PC71BM) photovoltaics for studying the saturation thresholds. The procedure introduced in this work paved the way for neutralizing the disadvantages of CNT precursors and simultaneously using their advantages in P3HT:PC71BM photovoltaics. The grafted-CNTs increased the capacity of active layers for acceptance of CNT and thus increased the cell performance (0.5 wt%, 4.09%). Interestingly, the pre-crystallized CNT/P3HT supramolecules eliminated the saturation point in P3HT:PC71BM solar cells, thereby the photovoltaic parameters were continuously raised even up to 50 wt%. The highest hole (μh) and electron (μe) mobilities belonging to P3HT:PC71BM:supramolecule devices (7.4 × 10−4 and 8.6 × 10−3 cm2/V.s) led to the highest photovoltaic characteristics (13.20 mA/cm2, 66% and 5.31%).

Effect of Post-Thermal Annealing on the Performance and Charge Photogeneration Dynamics of PffBT4T-2OD/PC71BM Solar Cells.

In this work, we studied influence of post thermal annealing on the performance and charge photogeneration processes of PffBT4T-2OD/PC71BM solar cells. As-prepared device exhibits a high-power conversion efficiency of 9.5%, much higher than that after thermal annealing. To understand this phenomenon, we studied charge photogeneration processes in these solar cells by means of time resolved spectroscopy. We associate the degradation of solar cell performance with the reduction of exciton dissociation efficiency and with increased bimolecular recombination of photogenerated charges as a result of annealing. We correlate the generation of localized PffBT4T-2OD polarons observed via spectro-electrochemical measurements with enhancement of the bimolecular charge recombination of annealed solar cells.

Fabrication and characterization of inverted organic PTB7:PC70BM solar cells using Hf-In-ZnO as electron transport layer

In this study, we report the use of Hafnium-Indium-Zinc-Oxide (HIZO) as electron transport layer (ETL) in inverted organic solar cells (iOSCs) using a bulk heterojunction of 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 [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) as active layer. The absorption spectrum and performance parameters obtained are compared to those obtained for similar iOSCs where the ETL was poly [(9,9-bis (30- (N,N-dimethylamino) propyl) -2,7-fluorene) -alt-2,7- (9,9-dioctylfluorene)] (PFN). Initially, devices showed the S-shaped current–voltage curve that has been modeled using a three-diode equivalent circuit model. This phenomenon can be removed by repeating electrical sweep-cycles under illumination. The degradation under nitrogen atmosphere and under encapsulation was also analyzed and compared with the degradation of the iOSCs using PFN. It was observed that the degradation of encapsulated HIZO-iOSC was twice slower than that of PFN-iOSCs. This increase in stability can be attributed to the Hf atoms, which have a stronger thermodynamic tendency to form metal oxides, suppressing the dissociation of HIZO.

Settled/unsettled blend nanofibers electrospun from photoactive polymeric/nonpolymeric constituents in PBDT‐DTNT:PCBM solar cells

ABSTRACTTo increase internal donor–acceptor interfaces and highlight the influence of ordering of donor–acceptor components inside fibers, novel blend fibrous structures comprising unsettled poly[benzodithiophene‐bis(decyltetradecyl‐thien) naphthothiadiazole] (PBDT‐DTNT):grafted‐reduced graphene oxide (rGO) nanofibers, settled PBDT‐DTNT/grafted‐rGO nanofibers including PBDT‐DTNT, and settled PBDT‐DTNT/grafted‐rGO nanofibers excluding PBDT‐DTNT were prepared and embedded in photovoltaics. Hence, three‐dimensional nonwoven network morphologies of triple electrospun fibers were acquired using electrospinning. Average diameter and conductivity of PBDT‐DTNT:grafted‐rGO, PBDT‐DTNT/grafted‐rGO:PBDT‐DTNT, and PBDT‐DTNT/grafted‐rGO fibers ranged in 200–250 and 1.1–1.6 × 10−9 S cm−1, 150–190 and 9.2–9.5 × 10−7 S cm−1, and 60–80 nm and 3.3–3.7 × 10−10 S cm−1, respectively. Photoluminescence quenching and thus donating–accepting characteristic of settled PBDT‐DTNT/grafted‐rGO nanofibers including PBDT‐DTNT were more intensified, resulting from greater internal interfaces. Through blending PBDT‐DTNT/grafted‐rGO supramolecules with PBDT‐DTNT chains and embedding them in PBDT‐DTNT:phenyl‐C71‐butyric acid methyl ester (PC71BM) thin films, the best results were obtained. Short‐circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (PCE) were 12.18 mA cm−2, 0.66 V, 65%, and 5.22%, respectively. Nanofiber template not only acted as guide path for charge transport but also increased interfacial area between donor and acceptor to induce more exciton dissociation. Inclusion of PBDT‐DTNT donor chains into blend nanofibers increased donor–acceptor interface in organic filaments. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47591.

Poly(N-isopropylacrylamide-co-methacrylic acid) Interfacial Layer for Efficient and Stable Inverted Organic Solar Cells

One of the key components in inverted organic solar cells is a zinc oxide (ZnO) layer as an electron-extraction layer. However, this layer contains electron traps that decrease the electron-extraction efficiency and reduce the photovoltaic performance. In this work, we report the photovoltaic property improvement of inverted PTB7-Th:PC71BM solar cells by coating high-molecular-weight poly(N-isopropylacrylamide-co-methacrylic acid) (H-PNIPAM) on top of the ZnO layer. The H-PNIPAM film thicknesses were carefully controlled by spin-coating different concentrations of H-PNIPAM solutions to generate an optimal thickness (3–5 nm). Atomic force microscopy and X-ray photoelectron spectroscopy revealed a uniformly coated H-PNIPAM layer. The photoluminescence spectra showed that the layer reduced the number of ZnO trap states. Contact angle measurements indicated that the layer modified the ZnO surface to become more hydrophobic, resulting in good contact with photoactive films. At the same time, the treatment decr...

Higher Mobility and Carrier Lifetimes in Solution‐Processable Small‐Molecule Ternary Solar Cells with 11% Efficiency

AbstractSolution‐processed small molecule (SM) solar cells have the prospect to outperform their polymer‐fullerene counterparts. Considering that both SM donors/acceptors absorb in visible spectral range, higher expected photocurrents should in principle translate into higher power conversion efficiencies (PCEs). However, limited bulk‐heterojunction (BHJ) charge carrier mobility (<10‐4 cm2 V‐1 s‐1) and carrier lifetimes (<1 µs) often impose active layer thickness constraints on BHJ devices (≈100 nm), limiting external quantum efficiencies (EQEs) and photocurrent, and making large‐scale processing techniques particularly challenging. In this report, it is shown that ternary BHJs composed of the SM donor DR3TBDTT (DR3), the SM acceptor ICC6 and the fullerene acceptor PC71BM can be used to achieve SM‐based ternary BHJ solar cells with active layer thicknesses >200 nm and PCEs nearing 11%. The examinations show that these remarkable figures are the result of i) significantly improved electron mobility (8.2 × 10‐4 cm2 V‐1 s‐1), ii) longer carrier lifetimes (2.4 µs), and iii) reduced geminate recombination within BHJ active layers to which PC71BM has been added as ternary component. Optically thick (up to ≈500 nm) devices are shown to maintain PCEs >8%, and optimized DR3:ICC6:PC71BM solar cells demonstrate long‐term shelf stability (dark) for >1000 h, in 55% humidity air environment.

Impact of Phosphorescent Sensitizers and Morphology on the Photovoltaic Performance in Organic Solar Cells

AbstractPhosphorescent sensitizers (PSs) are considered as a promising alternative for increasing the internal quantum efficiency (IQE) in organic solar cells (OSCs). By converting short‐lifetime singlet into long‐living triplet excitons, enhanced exciton diffusion and dissociation have been reported previously. However, only a limited increase in the OSC performance has been achieved. In this work, the interplay of the PS with both singlet and triplet excitons within organic blends is examined using kinetic Monte Carlo simulations including a comprehensive model of excitonic processes. Different morphologies of the conventional P3HT:PCBM solar cell are simulated, and the excitonic properties and their influence on the photovoltaic performance under doping are studied. The use of phosphorescent sensitization ensures high intersystem crossing and enlarges the diffusion length. An increase in the IQE of 34% is observed for a bilayer OSC. The increasing decay of triplets in proximity to the PS due to a strong spin‐orbit coupling limits the IQE. Unlike expected, triplet‐triplet annihilation does not provide a significant loss of excitons. A doped planar‐mixed molecular heterojunction outperforms an undoped bulk‐heterojunction OSC due to the enhanced exciton diffusion. A further study of optimal PS parameters predicts an increase in the IQE within bilayer solar cells by about 100%.

Role of geminate polaron-pair recombination on magnetoconductance in P3HT and PC71BM bulk-heterojunction organic solar cells

Abstract The mechanism of magnetoconductance (MC) in organic solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) remains unclear. Recombination of polaron pairs in the P3HT:PCBM organic solar cell is important in view of MC as well as charge transportation. To examine whether the polaron-pair model explains the mechanism of MC in the P3HT:PCBM system, we used an equivalent circuit model in which the internal shunt resistance represents the polaron-pair recombination. The almost identical dependence of the MC and the magnetic field effect on the internal shunt resistance for the P3HT:PCBM solar cells with various polaron-pair recombination rates clearly shows that the MC is ascribed to the polaron-pair recombination. Furthermore, the fact that the MC is virtually unaffected by light intensity below 100 mW/cm2 confirms that the polaron pairs responsible for the MC are geminate. The circuit model provides a useful instrument for probing the polaron-pair recombination in the system where excitons are effectively quenched.