PCBM Solar Cells Research Articles

Efficient P3HT:PC61BM solar cells employing 1,2,4-trichlorobenzene as the processing additives

The efficiency of the poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) based organic solar cells was enhanced by using 1,2,4-trichlorobenzene (TCB) as a processing additive to control the blend morphology. The addition of TCB improved the arrangement of P3HT which resulted in good phase separated blend films. Correspondingly, the optimized solar cells showed a power conversion efficiency (PCE) of 4.17% with a fill factor (FF) of 0.69, which were higher than those of common thermal annealing devices (PCE 3.84%, FF 0.67). The efficiency was further improved to 4.74% by thermal annealing at 150 °C for 10 min with a higher FF of 0.74.

Eco-friendly fabrication of PBDTTPD:PC71BM solar cells reaching a PCE of 3.8% using water-based nanoparticle dispersions

In this work we report on the eco-friendly processing of PBDTTPD:PC71BM organic solar cells using water-based nanoparticle (NP) dispersions. The polymer:fullerene NPs are prepared using the miniemulsion-solvent evaporation method, despite employing high-boiling solvents. Polymer solar cells are fabricated from these blend NPs and the device characteristics are studied in function of annealing time and temperature. The photoactive layer formation is carefully analyzed using atomic force microscopy (AFM). Annealing for longer times significantly increases the power conversion efficiency (PCE), up to 3.8%, the highest value reported for surfactant based NP solar cells. Our work shows that the low bandgap polymer PBDTTPD has the ability to afford reasonable efficiencies in NP solar cells in combination with PC71BM and paves the way to a truly eco-friendly processing of organic photovoltaics (OPVs).

Utilizing Forster resonance energy transfer to extend spectral response of PCDTBT:PCBM solar cells

Light harvesting in the near-infrared part of the solar spectrum is important to achieve high efficiency polymer solar cells (PSCs). In this work, we demonstrate that we take an existing polymer:fullerene blend and extend its spectral response into the near-IR region by adding a small amount near-IR absorbing dye in the blend. The polymer studied in this work is Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT). By adding only 2.5% squaraine dye to the blend, we were able to extend the spectral response of the resulting devices 100 nm into the near-IR spectral region. We show that the enhanced light harvesting is due to efficient Forster resonance energy transfer (FRET) between PCDTBT and the squaraine dye, resulting in an increase in power conversion efficiency. This type of ternary polymer solar cells is unique in that it allows the use of a small amount of selected dyes to extend light harvesting in infrared region.

D-A copolymers based on lactam acceptor unit and thiophene derivatives for efficient polymer solar cells

Two donor-acceptor (D-A) polymers PTTBDTP and PBTBDTP were developed by copolymerizing lactam acceptor [7,7′-bidithieno[3,2-b:2′,3′-d]pyridine]-5,5′(4H,4′H)-dione (BDTP) with thieno[3,2-b]thiophene (TT) or 2,2′-bithiophene (BT) in good yields. PTTBDTP and PBTBDTP exhibited excellent thermal stability and low HOMO energy levels of −5.52 eV ∼ −5.54 eV. Compared with PTTBDTP, PBTBDTP possessed broader absorption spectra, better crystallinity, and higher hole mobility. Therefore inverted polymer solar cells based on PBTBDTP:PC71BM achieved a higher PCE of 6.86%, with an open-circuit voltage (Voc) of 0.87 V, a short-circuit current (Jsc) of 10.97 mA/cm2 and a fill factor (FF) of 71.85%. Meanwhile, PTTBDTP:PC71BM solar cells gave a PCE of 5.84%. These results have demonstrated that lactam BDTP is a good building block for PSC applications.

Solution processed Cu2CdSnS4 as a low-cost inorganic hole transport material for polymer solar cells

The inefficient charge separation is one of the major impediments to improve the overall power conversion efficiency of the organic photovoltaics. Herein we demonstrate a new inorganic hole transporting materials for organic photovoltaics. Cu2CdSnS4 hole transport layer was deposited by solution based approach in low temperature by simple method. The thickness dependence of Cu2CdSnS4 hole transport layer on photovoltaic performance of organic solar cells was studied in details. The hole extraction activity of Cu2CdSnS4 thin films were compared with the reference PEDOT:PSS hole transport layer in the fabrication of P3HT:PCBM solar cells.

Model of Organic Solar Cell Photocurrent Including the Effect of Charge Accumulation at Interfaces and Non-Uniform Carrier Generation

We developed an improved model to fit the photocurrent density versus voltage in organic solar cells. The model has been validated by fitting data from P3HT:PCBM solar cells. Our model quantitatively accounts for the band bending near the electrodes caused by charge accumulation in the active layer. The model explains the position of the built-in and the zero-field voltage, the value of the internal electric field, the impact of electrode materials, and the appearance of multiple inflections. In addition, the model can be used to monitor the cell condition during accelerated lifetests.

Interfacial charge trapping in the polymer solar cells and its elimination by solvent annealing

The PCDTBT:PCBM solar cells were fabricated adopting a tandem layer approach to investigate the critical issues of charge trapping, radiation absorption, and efficiency in polymer solar cells. This layered structure was found to be a source of charge trapping which was identified and confirmed by impedance spectroscopy. The low efficiency in multilayered structures was related to trapping of photo-generated carriers and low carrier mobility, and thus an increased recombination. Solvent annealing of the structures in tetrahydrofuran vapors was found beneficial in homogenizing the active layer, dissolving additional interfaces, and elimination of charge traps which improved the carrier mobilities and eventually the device efficiencies.

Comprehensive method for analyzing the power conversion efficiency of organic solar cells under different spectral irradiances considering both photonic and electrical characteristics

The solar spectral irradiance varies significantly for different locations and time due to latitude, humidity, cosine effect of incident sunlight, etc. For convenience, the power-conversion efficiency of a solar cell is referenced to the international standard of AM1.5G spectral irradiance, which inevitably leads to varying performance of deployed solar cells under the specific local climate and insolation conditions. To predict the actual performance of solar cells under local climate conditions, we propose a methodology to compute the power-conversion efficiency of organic photovoltaic cells based upon indoor measurement with a solar simulator, the measured local solar spectrum, and making use of both optical and electrical factors. From our study, the annual average energy density yield of poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk-heterojunction organic solar cells under the local spectral irradiance of Malaysia is estimated to be 31.89kWh/m2 and the power-conversion efficiency is increased by 5.4% compared to that measured under AM1.5G conditions. In addition, diffuse solar irradiance (cloudy condition) was found to be in favor of P3HT:PCBM solar cells, with gain of 23.7–27.9% relative to AM1.5G conditions.

Enhance the light-harvesting capability of the ITO-free inverted small molecule solar cell by ZnO nanorods.

The ITO-free inverted SMPV1:PC71BM solar cells with an Al doped ZnO (AZO) transparent electrodes are fabricated. The AZO thin film prepared by pulsed laser deposition (PLD) technique exhibits high transmission (>85%) and low sheet resistance (~30 Ω/sq) and the power conversion efficiency (PCE) of devices based on AZO electrode can reach around 4%. To further enhance the light harvesting of the absorption layer of solar cells, ZnO nanorods interlayer is grown on the AZO layer before the deposition the active layer. The absorption spectrums of devices under various conditions are also simulated by RCWA method to identify the optical saturation length of the ZnO nanorods. The PCE of ITO-free inverted small molecule solar cell improved with ZnO nanorods can reach 6.6%.

Correlation between morphology and device performance of pBTTT:PC71BM solar cells

Phenyl-C71-butyric acid methyl ester (PC71BM) could intercalate between the side-chains of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT) creating a stable co-crystal structure, so optimal solar cell performance was obtained at high PC71BM concentrations promoting phase separated electron conductive pathways. Neutron reflectivity, with the application of magnetic contrast variation, was used to investigate the concentration profile of PC71BM within the active layer. And this profile was found to be homogeneous through the film thickness. Small angle neutron scattering was utilized to find there is amorphous PC71BM even at 50wt% of fullerene while previously it was believed that all fullerene was consumed to form co-crystals when its concentration is below 75wt%. These fullerene molecules evolve into approximately 15nm sized agglomerates to improve the electron transport when their concentration is above 75wt%. Thermal annealing gives these agglomerates mobility to form micrometer sized crystals causing a decrease of device performance. These findings can therefore correlate the morphology, especially in terms of fullerene agglomerates, vertical concentration profile and percolating structure they formed, with the device performance and provide valuable guidance for optimal morphology design of polymer: fullerene solar cells.

Fullerene‐Free Polymer Solar Cells with Open‐Circuit Voltage above 1.2 V: Tuning Phase Separation Behavior with Oligomer to Replace Polymer Acceptor

This study has proposed to use a well‐defined oligomer F4TBT4 to replace its analogue polymer as electron acceptor toward tuning the phase separation behavior and enhancing the photovoltaic performance of all‐polymer solar cells. It has been disclosed that the oligomer acceptor favors to construct pure and large‐scale phase separation in the polymer:oligomer blend film in contrast to the polymer:polymer blend film. This gets benefit from the well‐defined structure and short rigid conformation of the oligomer that endows it aggregation capability and avoids possible entanglement with the polymer donor chains. The charge recombination is to some extent suppressed and charge extraction is also improved. Finally, the P3HT:F4TBT4 solar cells not only output a high VOC above 1.2 V, but also achieve a power conversion efficiency of 4.12%, which is two times higher than the P3HT:PFTBT solar cells and is comparable to the P3HT:PCBM solar cells. The strategy of constructing optimum phase separation with oligomer to replace polymer opens up new prospect for the further improvement of the all‐polymer solar cells.

Revealing optically induced dipole-dipole interaction effects on charge dissociation at donor:acceptor interfaces in organic solar cells under device-operating condition

Organic materials normally have low dielectric constants and are often formed with high electron-hole binding energy, which are detrimental to the generation of photovoltaic actions in solution-processing thin-film solar cells. Here, we show that optically induced dipole-dipole interaction can largely decrease the electron-hole binding energy at donor:acceptor (D:A) based on the PTB7:PCBM bulk-heterojunctions. Our experimental measurements combine (i) double-beam 325nm and 532nm excitations to establish dipole-dipole interaction by selectively exciting the intramolecular charge-transfer donor and optically polarizable acceptor and (ii) magneto-photocurrent to monitor the electron-hole binding energy through charge dissociation at D:A interfaces. We find that the electron-hole binding energy at D:A interfaces can be significantly decreased when the dipole-dipole interaction is optically established in PTB7:PCBM solar cells. Furthermore, the dipole-dipole interaction forms a drifting field to facilitate charge transport, and consequently enhancing the Voc and FF in developing photovoltaic actions in organic solar cells.

Optical absorption study of P3HT:PCBM blend photo-oxidation for bulk heterojunction solar cells

We investigated the photo-oxidation of the poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C-butyric acid methyl ester (PCBM) blend thin films in ambient air by ultraviolet–visible spectrophotometry in the differential mode. The decomposition of the difference absorbance spectra showed with the exposure time a successive decrease of the excitonic absorption (S0) at ∼605nm corresponding to a singlet exciton generation. This decrease is accompanied with a decline of the absorption bands at ∼558nm and ∼509nm connected with one exciton plus one phonon and one exciton plus two phonons generation, respectively. Simultaneously, there is observed an increase of the sub-bandgap absorbance (SGap) in the wavelength interval of 640–800nm. Though the observed changes were in the order of thousandth of the pristine absorbance, these changes caused a dramatic degradation of the organic solar cell performance mainly due to a drop of the short circuit current (JSC). The correlation between the absorbance features S0, SGap and the JSC of the P3HT:PCBM solar cell implies that the S0 decrease is too low to explain such a dramatic change of the JSC induced by the photo-oxidation. On the contrary, the experimental dependence of the JSC on the SGap is reasonable explained with the increase of the exciton recombination at defect states visible via the SGap.

Enhancement of Photovoltaic Performance by Utilizing Readily Accessible Hole Transporting Layer of Vanadium(V) Oxide Hydrate in a Polymer-Fullerene Blend Solar Cell.

Herein, a successful application of V2O5·nH2O film as hole transporting layer (HTL) instead of PSS in polymer solar cells is demonstrated. The V2O5·nH2O layer was spin-coated from V2O5·nH2O sol made from melting-quenching sol-gel method by directly using vanadium oxide powder, which is readily accessible and cost-effective. V2O5·nH2O (n ≈ 1) HTL is found to have comparable work function and smooth surface to that of PSS. For the solar cell containing V2O5·nH2O HTL and the active layer of the blend of a novel polymer donor (PBDSe-DT2PyT) and the acceptor of PC71BM, the PCE was significantly improved to 5.87% with a 30% increase over 4.55% attained with PSS HTL. Incorporation of V2O5·nH2O as HTL in the polymer solar cell was found to enhance the crystallinity of the active layer, electron-blocking at the anode and the light-harvest in the wavelength range of 400-550 nm in the cell. V2O5·nH2O HTL improves the charge generation and collection and suppress the charge recombination within the PBDSe-DT2PyT:PC71BM solar cell, leading to a simultaneous enhancement in Voc, Jsc, and FF. The V2O5·nH2O HTL proposed in this work is envisioned to be of great potential to fabricate highly efficient PSCs with low-cost and massive production.

Compatibility of PTB7 and [70]PCBM as a Key Factor for the Stability of PTB7:[70]PCBM Solar Cells

The rapid degradation of organic photovoltaic (OPV) devices compared to conventional inorganic solar cells is one of the critical issues that have to be solved in order to make OPV a competitive commercial technology. The understanding of the fundamental mechanisms that reduce the power conversion efficiency (PCE) over time is beneficial for the design of new materials with enhanced stability. This paper focuses on bulk heterojunction organic solar cells based on thieno [3,4‐b] thiophene‐alt‐benzodithiophene (PTB7) mixed with [6,6]‐phenyl‐C71‐butyric acid methyl esther ([70]PCBM). In spite of being promising in terms of PCE, devices based on this blend are unstable and have a short lifetime. When exposed to light in inert atmosphere, the PCE drops by 15% in less than 1 h and by 35% in 8 h; this degradation is induced by the ultraviolet (UV) part of the spectrum. This paper analyzes the effect induced by UV light on the transport of charges in PTB7:[70]PCBM. Contrary to expectations, the electron transport shows evidence of trapping, while the transport of holes appears unaffected. Furthermore, it is proven that the loss of PCE is due to a reaction between PTB7 and [70]PCBM, while the intrinsic instability of the polymer plays a marginal role.

Dramatically enhanced performances and ideally controlled nano-morphology via co-solvent processing in low bandgap polymer solar cells

The device performance of photovoltaics with a polymer:fullerene bulk heterojunction (BHJ) structure, consisting of DT-PDPP2T-TT donor polymer and poly(3-hexylthiophene):[6,6]phenyl-C61-butyric acid methyl ester (PC61BM) acceptor compound, was investigated as a function of co-solvent composition. An enhancement of the photocurrent density and fill factor is observed in diodes made by spin-coating with chloroform mixed with ortho-dichlorobenzene, which allows a significantly higher device efficiency of 5.55% compared to diodes made from neat chloroform (efficiency = 3.61%). To clarify the role of the co-solvent, we investigated the nanoscale morphology with AFM, TEM and 2D-GIWAXS techniques and also the free-charge carrier mobility via space-charge limited current theory. We obtained the result that, under such supersaturated conditions, co-solvents induce increased polymer crystalline aggregation into a 3D phase structure and boost charge-carrier transport characteristics. This provides a rational basis for the development of ideally-controlled BHJ films that yield efficient DT-PDPP2T-TT:PCBM solar cells. Therefore, carefully selecting solvent mixtures provides an approach toward efficient low bandgap polymer solar cells.

Investigation of the magnetic nickel nanoparticle on performance improvement of P3HT:PCBM solar cell

This work reports an investigation on the performance improvement of bulk heterojunction polymer solar cells made of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) blend system by introducing nickel nanoparticles (Ni NPs). Using the simplest cell structure, the addition ratio of 2 % of Ni NPs results in a remarkable increase in the light absorption and the short current, which brings an enhancement on power conversion efficiency with 25 % compared with the reference device. The analysis indicates that the electromagnetic surface wave generated from Ni NPs coupled with the photoactive layer forms surface plasmon resonance, which can result in a light trapping effect to increase the light absorption. As a conclusion, the inexpensive Ni NPs may provide an effect and alternative process on the performance improvement of the polymer solar devices.

Bulk heterojunction perovskite–PCBM solar cells with high fill factor

An inverted bulk heterojunction perovskite–PCBM solar cell with a high fill factor of 0.82 and a power conversion efficiency of up to 16.0% was fabricated by a low-temperature two-step solution process. The cells exhibit no significant photocurrent hysteresis and their high short-circuit current density, fill factor and efficiency are attributed to the advantageous properties of the active layer, such as its high conductivity and the improved mobility and diffusion length of charge carriers. In particular, PCBM plays a critical role in improving the quality of the light-absorbing layer by filling pinholes and vacancies between perovskite grains, resulting in a film with large grains and fewer grain boundaries. Bulk heterojunction perovskite solar cells with a high fill factor are reported.

Alternative selection of processing additives to enhance the lifetime of OPVs

The use of processing additives is known to accelerate the degradation of organic photovoltaics (OPVs) and therefore, this paper studies the impact of selecting alternative processing additives for PCPDTBT:PC71BM solar cells in order to improve the stability. The use of naphthalene-based processing additives has been undertaken, which is shown to reduce the initial power conversion efficiency by 23%–42%, primarily due to a decrease in the short-circuit current density, but also fill factor. However, the stability is greatly enhanced by using such additives, with the long term stability (T50%) enhanced by a factor of four. The results show that there is a trade-off between initial performance and stability to consider when selecting the initial process additives. XPS studies have provided some insight into the decreased degradation and show that using 1-chloronaphthalene (ClN) leads to reduced morphology changes and reduced oxidation of the thiophene-ring within the PCPDTBT backbone.

VOC enhancement in polymer solar cells with isobenzofulvene–C60 adducts

We report the use of isobenzofulvene–C60 adducts in bulk heterojunction organic solar cells, synthesized via the [4 + 2] cycloaddition of C60 with an in situ generated isobenzofulvene intermediate. The LUMO energy levels of these adducts are 20–180 meV higher than that of PCBM ([6,6]-phenyl-C61-butyric acid methyl ester). This large increase of the LUMO level is attributed to cofacial π-orbital interactions between the fullerene surface and the isobenzofulvene π–system (aromatic ring and double bond). Raised LUMO levels of fullerenes, together with their desirably slow recombination dynamics, led to higher open-circuit voltages (VOC) in bulk heterojunction polymer solar cells (up to 0.75 V for bisadducts) relative to cells tested in parallel using the well-known PCBM as the fullerene acceptor. In addition to enhanced VOC, the short-circuit current densities (JSC) were improved in the devices containing the epoxide analogs of the isobenzofulvene–C60. Notably the epoxide derivative of the monoadduct (IBF–Ep) exhibited ∼20% enhancement of power conversion efficiency (PCE) compared to reference P3HT:PCBM solar cells. A combination of optical and electronic methods was used to investigate the origin of the PCE enhancement observed with these new fullerene acceptors with particular attention to the increased VOCs.