Planar Heterojunction Organic Photovoltaics Research Articles

Device Modelling and Optimization of Nanomaterial-Based Planar Heterojunction Solar Cell (by Varying the Device Dimensions and Material Parameters).

The objective of this work is to model a multi-disciplinary (multi-physics) organic photovoltaic (OPV) using mathematical modeling and analyzing the behavior of a standard planar heterojunction (PHJ) or bi-layer thin-film photovoltaic device, supporting the optimization of an efficient device for future production and assisting in evaluating and choosing the materials required for the efficient device. In order to increase photodiode performance, the device structure and geometrical properties have also been optimized and evaluated. In this work, the effects of varying the device size and transport parameters on the performance parameters of a PHJ OPV structure comprised of Indium Tin Oxide as the anode (ITO), semiconducting single-wall carbon nanotube (s-SWCNT) as the donor, fullerene C70 as the acceptor, and Aluminium (Al) as the cathode have been analyzed. The conclusion suggests that a highly effective ITO/s-SWCNT/C70/Al PHJ solar cell may be fabricated if the suggested device is appropriately built with a thin layer and a high exciton diffusion length, bi-molecular recombination coefficient, and improved mobility charge carriers, in particular hole mobility in the cell’s donor layer. In addition, the displayed current–voltage (I–V) characteristics of the proposed PHJ device are clearly indicated, with the ITO/s-SWCNT/C70/Al combination having the greatest short-circuit current density (Jsc) value of 5.61 mA/cm2, open-circuit voltage (Voc) of 0.7 V, fill factor (FF) of 79% and efficiency (ɳ) of 3.1%. Results show that the electrical performance of organic solar cells is sensitive to the thickness of the photoactive substance. These results open the path for developing inexpensive and highly efficient solar cells.

Investigating the Indoor Performance of Planar Heterojunction Based Organic Photovoltaics

Recently, the bulk heterojunction (BHJ)-based photoactive layer has been favored over the planar heterojunction (PHJ)-based photoactive layer for organic photovoltaics (OPVs), owing to the evitable thickness limit of the latter induced by the low mobility of charge carriers. However, under dim indoor light conditions, the PV performance becomes immune to the resistive components of the PHJ OPVs, such as a thick photoactive layer, thereby rendering them attractive for operating indoors. In this study, we devise and demonstrate indoor OPVs with a PHJ-based photoactive layer constituting poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C60 bisadduct (ICBA). The performance of the PHJ OPVs was optimized by modulating the thickness of the ICBA layer under a 1000 lx light-emitting diode lamp. OPVs with a 65 nm-thick ICBA layer exhibited excellent indoor performance with a power conversion efficiency of 15.3 ± 0.3%, comparable to that of the reference BHJ OPVs (15.0 ± 0.2%). No noticeable performance degradation was observed by increasing the thickness of the PHJ-based photoactive layer; instead, a thick photoactive layer only enhanced the light absorption, resulting in excellent indoor performance with a higher photocurrent.

Initial Engineering and Outdoor Stability Assessment of "Gray/Black" Fullerene-Free Organic Photovoltaics Based on Only Two Complementary Absorbing Materials: A Tetrabenzotriazacorrole and a Subphthalocyanine.

Broad absorption is a desired characteristic of materials employed in the photoactive layers of organic photovoltaic (OPV) devices. Here, we have identified tetrabenzotriazacorroles (Tbcs) as complementary absorbing chromophores and electron donors to the promising nonfullerene acceptors boron subphthalocyanines (BsubPcs). These two materials, which can be utilized as donor–acceptor pairs within fullerene-free OPVs, yield spectral coverage over the entire visible range of 300–750 nm. Oxy phosphorus Tbc derivative (POTbc) was employed as an electron donor and paired initially with multiple BsubPc derivatives having a distribution of highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels in planar heterojunction OPVs. These devices were “gray/black” due to the broad absorption across the visible spectrum. Upon screening, the partially halogenated chloro hexachloro BsubPc (Cl–Cl6BsubPc) showed the greatest promise for coupling with POTbc. The thickness ratio and total thickness of the active layer were then probed in order to identify the optical and electrical limitations on the POTbc/Cl–Cl6BsubPc-based OPV device. A maximum power conversion efficiency (PCE) of 2.13% was achieved at 60 nm total thickness of the active layer and 1 to 3 (POTbc to Cl–Cl6BsubPc) thickness ratio. Outdoor stability of the champion device was evaluated using protocols established by International Summits on OPV Stability and was found to be on par with an α-sexithiophene/Cl–Cl6BsubPc baseline OPV.

Phenoxy-(Chloro) n -Boron Subnaphthalocyanines: Alloyed Mixture, Electron-Accepting Functionality, and Enhanced Solubility for Bulk Heterojunction Organic Photovoltaics.

The first set of phenoxy BsubNc compounds, PhO-ClnBsubNc and F5-ClnBsubNc, was synthesized through an axial displacement reaction of Cl-ClnBsubNc with phenol and pentafluorophenol (respectively). Like their precursor, the products were found to be an alloyed mixture of phenoxylated ClnBsubNcs with random positioning in the solid state yet consistent frequency of bay position chlorination. The average bay position chlorine occupancy was determined to be 0.99 through single crystal diffraction of PhO-ClnBsubNc. Although the phenoxylation of Cl-ClnBsubNc did not influence the chromophore photophysical properties, the electrochemical behavior was found to be more stable. Phenoxylation yielded differences in organic photovoltaic (OPV) device metrics. Specifically, a significant increase in open circuit voltage (VOC) was observed, ultimately exceeding 1.0 V when phenoxylated ClnBsubNcs were paired with alpha-sexithiophene (α-6T) in planar heterojunction OPVs. Phenoxylation enabled the first example of BsubNcs incorporated into polymer-based bulk heterojunction (BHJ) OPVs through enhanced solubility. Phenoxylated ClnBsubNcs, when paired with poly-3-hexylthiophene, also showed high VOC in BHJ OPVs with broad spectral absorption up to 760 nm. In the BHJ case, simple phenoxy was shown to be a better axial substituent compared to pentafluorophenoxy. This study represents the first example of using ClnBsubNcs with nonchlorine axial substituents in OPVs and demonstrates that phenoxylation has a significant impact on device metrics while enhancing solubility to enable incorporation of ClnBsubNcs into BHJ OPVs.

Outdoor Performance and Stability of Boron Subphthalocyanines Applied as Electron Acceptors in Fullerene-Free Organic Photovoltaics

The outdoor lifetime and performance of organic photovoltaics (OPVs) using boron subphthalocyanine (BsubPc) derivatives as electron-accepting materials is presented. The protocols followed are based on the most advanced level of outdoor testing established by the International Summit on OPV Stability (ISOS). The stability of each BsubPc is compared using three different sets of encapsulated planar heterojunction OPVs, with each set containing a different BsubPc as the electron-accepting layer. The performance and stability of each set is tested outdoors using an epoxy glue and a glass coverslip as protection from the ambient environment. Outdoor testing continued until the OPVs reached 80 or 50% of their original power conversion efficiency, as determined by frequent indoor characterization. OPVs utilizing chloro-BsubPc are shown to exhibit the highest stability and performance, while the stability of the other two BsubPc derivatives is reduced presumably as a result of their phenoxy or phenyl functionali...

Oxy phosphorus tetrabenzotriazacorrole: firming up the chemical structure and identifying organic photovoltaic functionality to leverage its unique dual absorbance

Tetrabenzotriazacorroles are a family of molecules that exhibits dual absorbance in the visible spectrum. Here we identify their functionality in planar heterojunction organic photovoltaics for the first time.

Experimental evidence of harmful exciton dissociation at MoO3/CuPc interface in OPV

Organic photovoltaics (OPVs) with three types of double anode buffer layers (DABLs), i.e., 4.5 nm hole-transport material 4,4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl layer, 1 nm electron-transport material Bphen, and 1 nm typical insulator LiF layer, respectively, deposited onto 10 nm MoO3 layer, were fabricated. All these three DABLs can improve the efficiency of CuPc/C60 based planar heterojunction OPV, especially with about 10% enhancement of short-circuit current (ISC). Based on the external quantum efficiency (EQE) and transient photovoltage (TPV) measurements, a mechanism of depressing harmful exciton dissociation at the MoO3/CuPc interface has been proposed. This harmful dissociation results in exciton loss within the CuPc layer, while a proper ultrathin layer inserted at MoO3/CuPc interface can effectively depress the dissociation and thus improve the total photocurrent.

P-n interface stabilization of planar heterojunction organic photovoltaics by an ethyleneoxy side chain of methanofullerenes

Methanofullerenes bearing both an ethyleneoxy side chain, −(CH2CH2O)n-, n=1 or 2, and a 5-ethyl-2-thienyl group were prepared. These methanofullerenes were much more polar than PCBM and C60, and solubility in chlorobenzene (23–24g/L) was comparable to that of PCBM. Planar heterojunction photovoltaic cells employing the methanofullerenes as an electron acceptor and copper phthalocyanine (CuPc) as an electron donor were fabricated. The short-circuit current density of the photovoltaic cell (4.61–4.66mA/cm2) was larger than that of the photovoltaic cell prepared from the standard fullerene acceptor, PCBM, (4.08mA/cm2), and consequently the power conversion efficiency was improved. Comparisons of contact angles of water and hexadecane on fullerene films indicated that the fullerenes with an ethyleneoxy side chain formed more stable interface with CuPc than PCBM. These results suggest that side chain engineering aiming at stable p-n interface is an attractive strategy for designing efficient organic photovoltaic cells.

Energy Level Tuning of Non-Fullerene Acceptors in Organic Solar Cells.

The use of non-fullerene acceptors in organic photovoltaic (OPV) devices could lead to enhanced efficiencies due to increased open-circuit voltage (VOC) and improved absorption of solar light. Here we systematically investigate planar heterojunction devices comprising peripherally substituted subphthalocyanines as acceptors and correlate the device performance with the heterojunction energetics. As a result of a balance between VOC and the photocurrent, tuning of the interface energy gap is necessary to optimize the power conversion efficiency in these devices. In addition, we explore the role of the charge transport layers in the device architecture. It is found that non-fullerene acceptors require adjusted buffer layers with aligned electron transport levels to enable efficient charge extraction, while the insertion of an exciton-blocking layer at the anode interface further boosts photocurrent generation. These adjustments result in a planar-heterojunction OPV device with an efficiency of 6.9% and a VOC above 1 V.

Improved power conversion efficiency of bulk-heterojunction organic photovoltaic cells using neat C70 as an effective acceptor for an amorphous π-conjugated polymer

Low-cost neat C70 was successfully utilized to fabricate solution-processable bulk heterojunction (BHJ) organic photovoltaics (OPVs) by combining the neat C70 with an amorphous polymer, PEDOTNDIF, which was also synthesized at low cost by using a two-step direct arylation reaction. The optimized BHJ OPV using PEDOTNDIF:C70 exhibited a short-circuit current density (Jsc) of 8.70mAcm−2, an open-circuit voltage of 0.75V, a fill factor of 0.36, and a power conversion efficiency (PCE) of 2.33%, as compared to 1.46mAcm−2, 0.80V, 0.26, and 0.30%, respectively, for the BHJ OPV fabricated using PEDOTNDIF:PC70BM. The much-improved PCE (Jsc) is mainly due to the strong acceptor nature of neat C70, which needs to efficiently separate photo-excitons into free carriers. A good miscibility in the PEDOTNDIF:C70-based BHJ film with a large donor/acceptor interfacial area was also confirmed by characterizing atomic force microscopy images of the BHJ films and comparing PCEs of planar-heterojunction OPVs with PEDOTNDIF/C70 bilayers.

Tuning Open-Circuit Voltage in Organic Solar Cells with Molecular Orientation.

The role of molecular orientation of a polar conjugated polymer in polymer-fullerene organic photovoltaic (OPV) cells is investigated. A planar heterojunction (PHJ) OPV cell composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is used as a model system to isolate the effect of the interfacial orientation on the photovoltaic properties. The molecular orientation of the aggregate P3HT relative to the PCBM layer is varied from highly edge-on (conjugated ring plane perpendicular to the interface plane) to appreciably face-on (ring plane parallel to the interface). It is found that as the P3HT stacking becomes more face-on there is a positive correlation to the OPV open-circuit voltage (V(OC)), attributed to a shift in the highest occupied molecular orbital (HOMO) energy level of P3HT. In addition, the PHJ OPV cell with a broad P3HT stacking orientation distribution has a V(OC) comparable to an archetypal bulk heterojunction (BHJ) device. These results suggest that, in the BHJ OPV cell, the hole energy level in the charge transfer state is defined in part by the orientation distribution of the P3HT at the interface with PCBM. Finally, the photoresponses of the devices are also shown to have a dependence on P3HT stacking orientation.

Synthesis of Novel Push–Pull Chromophores based on N-Ethylcarbazole for Vacuum Deposition Processed Organic Photovoltaics

Abstract Two novel p-type materials, Cz(Et)ThOM and Cz(Et)OM—respectively with and without a thiophene π-spacing unit—have been synthesized for vacuum-deposited organic photovoltaics (OPVs). The power conversion efficiencies of planar heterojunction OPVs of the former are clearly higher than that of the latter. Further improvement of the device with Cz(Et)ThOM was obtained by employing bulk heterojunction structure and post-annealing. The structural analysis of thin films by synchrotron X-ray diffraction and atomic force microscopy shows that this improvement is due to Cz(Et)ThOM becoming highly ordered during annealing.

Dominant effects of first monolayer energetics at donor/acceptor interfaces on organic photovoltaics.

Energy levels of the first monolayer are manipulated at donor/acceptor interfaces in planar heterojunction organic photovoltaics by using molecular self-organization. A "cascade" energy landscape allows thermal-activation-free charge generation by photoirradiation, destabilizes the energy of the interfacial charge-transfer state, and suppresses bimolecular charge recombination, resulting in a higher open-circuit voltage and fill factor.

A new anodic buffer layer material for non-mixed planar heterojunction chloroboron subphthalocyanine organic photovoltaic achieving 96% internal quantum efficiency

Nonmixed planar heterojunction (PHJ) small-molecule organic photovoltaics (OPVs) with 96% internal quantum efficiency (at 595nm) and 4.77% power conversion efficiency (PCE) have been demonstrated. In addition to boron subphthalocyanine chloride (SubPc) and C60 as electron donor and acceptor materials, respectively, PHJ OPVs contain an ultrathin (2nm) buffer layer of bis-(naphthylphenylaminophenyl)fumaronitrile (NPAFN) between the indium tin oxide (ITO) anode and the donor layer (SubPc). Compared with copper phthalocyanine (CuPc) or α-naphthylphenylbiphenyl diamine (NPB) buffer layers, the NPAFN buffer layer blocks the exciton diffusion from the SubPc electron donor layer to the ITO anode more effectively and considerably improves the short circuit current (JSC) from 5.96 (without an NPAFN layer) to 7.70mA/cm2 (with a 4-mm-thick NPAFN layer ). In addition, experimental results indicated that the NPAFN buffer layer reduces the crystallization, or stacking, of the SubPc electron donor, thereby limiting the reverse saturation current and elevating the open circuit voltage (VOC) from 1.01 (without an NPAFN layer) to 1.08V (with a-2-nm thick NPAFN layer). However, series resistance (RS) of the OPV monotonically increases with increasing NPAFN layer thickness. The performance of the OPV is optimized when the NPAFN buffer layer thickness is 2nm. Compared with a SubPc–C60 PHJ OPV without an NPAFN buffer layer, the PCE of a OPV with a buffer layer increases by 22% from 3.96% to 4.77%, with a concurrent increase in JSC (from 5.96 to 7.02mA/cm2) and VOC (from 1.01 to 1.08V). However, a decrease in RS (from 10.21 to 14.95Ωcm2) and in fill factor (from 65% to 63%) is also observed.

Energy Level Alignment and Morphology of Ag and Au Nanoparticle Recombination Contacts in Tandem Planar Heterojunction Solar Cells

We present the interface characterization of vacuum-deposited metal nanoparticle recombination layers (Ag, Au; 1 nm equivalent thickness) at donor/acceptor heterojunctions comprising copper phthalocyanine (CuPc) and C60 as model interfaces for tandem planar heterojunction organic photovoltaics (TOPVs). We compare the extent to which voltage doubling occurs using these two metal recombination contacts (RC) in TOPVs (ITO/CuPc/C60/(Ag,Au)/CuPc/C60/BCP/Al) and correlate the differences with energetic dissimilarities revealed by UV-photoemission (UPS) and inverse photoemission (IPES) spectroscopies, and morphology as revealed by atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). Ag interlayer RCs produce the expected voltage doubling in the open-circuit voltage (VOC) for the TOPV, whereas Au RCs showed poor voltage addition. Significant shifts in ionization potential and electron affinity and shifts in local work function were observed for C60/metal heterojunctions and for heterojunctions based on C60/metal/C60 and for C60/metal/CuPc, with clear evidence for partial charge redistribution between C60 and Ag nanoparticles. AFM and FE-SEM images revealed discrete Ag nanoparticles at the C60 interface, whereas Au/C60 heterojunctions consisted of more uniform Au thin films that wet the C60 surface and penetrated below the surface. These studies point to the need for careful control of both electronic and morphological properties of thin RCs in emerging tandem organic solar cell technologies.

Recovering lost excitons in organic photovoltaics using a transparent dissociation layer

In organic photovoltaic (OPV) cells, photocurrent generation relies on exciton diffusion to the donor/acceptor heterojunction. Excitons that fail to reach the heterojunction are lost to recombination via quenching at the electrodes or relaxation in the bulk. Bulk recombination has been mitigated largely through the use of bulk heterojunctions, while quenching at the metal cathode has been previously circumvented through the introduction of exciton blocking layers that “reflect” excitons. Here, we investigate an alternative concept of a transparent exciton dissociation layer (EDL), a single layer that prevents exciton quenching at the electrode while also providing an additional interface for exciton dissociation. The additional heterojunction reduces the distance excitons must travel to dissociate, recovering the electricity-generating potential of excitons otherwise lost to heat. We model and experimentally demonstrate this concept in an archetypal subphthalocyanine/fullerene planar heterojunction OPV, generating an extra 66% of photocurrent in the donor layer (resulting in a 27% increase in short-circuit current density from 3.94 to 4.90 mA/cm2). Because the EDL relaxes the trade-off between exciton diffusion and optical absorption efficiencies in the active layers, it has broad implications for the design of OPV architectures and offers additional benefits over the previously demonstrated exciton blocking layer for photocurrent generation.

Optical analysis of organic photovoltaic cells incorporating graphene as a transparent electrode

The optical performance of organic photovoltaic (OPV) cells incorporating graphene as a transparent anode is investigated using the transfer matrix method coupled with the exciton diffusion equation, and is also compared with ITO based devices. It turns out that graphene based OPVs can realize 7–10% higher Jsc values in planar heterojunction OPVs and ∼5% higher total number of absorbed photons in bulk heterojunction OPVs than the ITO based OPVs, depending on the absorption wavelength of the OPVs. The improvement originates from the low reflective loss of the graphene electrode compensating for the high absorption by the electrode to increase the absorption by the active layers. Since the graphene electrode is flexible and can achieve low sheet resistance with a very thin layer, the graphene has a large potential to fully replace the ITO anode in OPVs, especially for flexible devices.

Correlation between energy level alignment and device performance in planar heterojunction organic photovoltaics

Information on the interfacial electronic structure in organic photovoltaics (OPVs) is essential for fully understanding features of device operation such as the photocurrent generation and relative energy band offsets at the donor/acceptor interface, which directly affect the open circuit voltage (Voc). Kelvin probe (KP) measurements fully reveal the energy level alignment in a prototype OPV with a copper phthalocyanine (CuPc)/fullerene (C60) planar heterojunction. Energy level pinning at the CuPc/C60 junction fixes the energy band offsets of C60. A downward energy shift of about 0.9eV appears at the C60/bathocuproine junction, which may act as a hole-blocking barrier. A combination of KP and current density–voltage measurements indicates that photocurrent generation depends strongly on the magnitude of the upward energy shift at the CuPc/C60 junction. The dependence of Voc on the substrate work function is also discussed in terms of the energy level alignment at indium tin oxide/CuPc/C60 junctions.

Orientation management of α-sexithiophene layer for the application in organic photovoltaic devices

Management of α-sexithiophene (6T) molecular orientation is a key issue to improve the performance of organic photovoltaic (OPV) devices based on 6T thin films, which act as p-type semiconductor layers; however, it is difficult to produce 6T layers with the molecules lying on smooth surfaces, including oxide covered substrates. We have succeeded in orienting the 6T molecules on oriented conductive polythiophene (PT) films, parallel to the oriented PT molecular axis; i.e. parallel to the substrate, by evaporating 6T in vacuum. Here, we reported planar heterojunction (PHJ) OPV devices consisting of 6T films with molecular orientation parallel to the substrate. The orientation of 6T led to improvement of the power conversion efficiency (PCE) in these PHJ OPVs. The average PCE of PHJ OPVs with oriented PT was 2.8times higher than that without PT. The PCE and the maximum value of the short-circuit photocurrent action spectra of the OPV with the PT was larger under irradiation of polarized light parallel to the 6T molecular axis than that orthogonal to the axis. On the other hand, the PCE and the action spectra of the OPV without the PT did not show the response to polarized light.

Comparison of short and long wavelength absorption electron donor materials in C60-based planar heterojunction organic photovoltaics

Buckminsterfullerene, C60-based planar heterojunction (PHJ) organic photovoltaics (OPVs) have been created using a short wavelength absorption (λmax=490nm) electron-donating bis(naphthylphenylaminophenyl)fumaronitrile (NPAFN). NPAFN exhibits a hole mobility greater than 0.07cm2V−1s−1 as determined by its field-effect transistor. It can be attributed to such hole mobility that enables a thin layer (<10nm) NPAFN in PHJ OPV, ITO/NPAFN/C60/bathocuproine/Al. Because of the low lying HOMO energy level (5.75eV) of NPAFN and relatively high ionization potential ITO (∼5.58eV), such OPVs exhibit a very high open circuit voltage of ∼1.0V, relatively high fill factor of 0.60, and a relatively high shunt resistance of 1100Ωcm−2, which all compensate for a relatively low short circuit current of 3.15mAcm−2 due to the short absorption wavelength and inferred short exciton diffusion length of NPAFN. Altogether, NPAFN OPVs display a power conversion efficiency (ηPC) of 2.22%, which is better than other long wavelength absorption materials in similar PHJ OPVs, such as pentacene (λmax 670nm, HOMO 5.12eV, ηPC 1.50%) and copper phthalocyanine (λmax 624, 695nm, HOMO 5.17eV, ηPC 1.43%).