Tandem Organic Photovoltaic Cells Research Articles

Understanding tandem organic photovoltaic cell performance

We develop a framework to understand the performance of tandem organic photovoltaic (OPV) cells consisting of a series-connected stack of an arbitrary number of sub-cells. The power conversion efficiency penalty, Δη, is defined as the loss incurred when the tandem cell is at its maximum power point (MPP) but one or more sub-cells are not operating at their individual MPPs. To minimize Δη, the current at the MPP for each sub-cell must be equal. We also develop a method to calculate the tandem cell spectral mismatch factor and fill factor, showing that they are related to both the fill factors and short circuit currents of all the constituent sub-cells. By including the current generated in the dark, exciton dissociation at the donor-acceptor heterojunction, and photoconductivity, along with current losses due to polaron-pair and bimolecular recombination, we simulate the operation of small molecule bilayer and mixed-layer sub-cells used in the tandem, and from these results derive the behavior of the integrated device. This analysis is used to understand and optimize tandem OPV cell performance.

An efficient interconnection unit composed of electron-transporting layer/metal/p-doped hole-transporting layer for tandem organic photovoltaics

We report an efficient interconnection unit (ICU) consisting of an electron transporting layer/metal/p-doped hole transporting layer (p-HTL) structure for tandem organic photovoltaic (TOPV) cells. The ICU satisfies all the requirements of optical transparency and low voltage loss and for functioning as an optical spacer. The variation of the short circuit current and open circuit voltage (VOC) of the TOPV cells with increasing thickness of the p-HTL in the ICU followed the theoretical predictions, proving that the ICU does not disturb the electrical characteristics of the TOPV cells up to a p-HTL thickness of 100 nm with minimal VOC loss (∼3%).

Microcavity tandem solar cells with a short circuit current higher than single cells

We demonstrate by numerical simulation that microcavity tandem organic photovoltaic cells (OPVs) with a capping layer can have a higher short circuit current (Jsc) than normal ITO based tandem OPVs without the microcavity structure and corresponding single cells. A microcavity tandem OPV consists of SubPc–C60 and PbPc–C60 donor–acceptor pairs of each subcell. Two reflecting electrodes, spacer layers, and a capping layer in the microcavity structure control the optical electric field of the device so that Jsc in the tandem OPV becomes higher than the normal cells even in its thinner active layers. The optimized microcavity device leads to a 25% increase in Jsc compared with the conventional ITO-based device employing the same donor–acceptor pair, and a 5% increase in Jsc compared with conventional ITO-based single cells.

Tandem organic photovoltaics using both solution and vacuum deposited small molecules

We demonstrate a tandem organic photovoltaic cell incorporating solution- and vacuum-deposited small molecules as the active layers. A blue and green-absorbing boron subphthalocyanine chloride:C70 graded heterojunction (HJ) sub-cell is combined with a green and red-absorbing functionalized squaraine/C70 bilayer HJ sub-cell, resulting in a tandem cell with a wavelength response from 350 nm to 800 nm. The efficiency of the cells depends on process conditions such as solvent annealing, resulting in nanocrystalline morphology that leads to improved charge and exciton transport compared with un-annealed cells. The incorporation of C70 in both sub-cells leads to an increase of short-circuit current by at least 30% compared to analogous cells using C60. The optimized power conversion efficiency of the tandem cell is 6.6% ± 0.1%, with an open-circuit voltage of 1.97 ± 0.1 V under simulated 1 sun, AM 1.5G illumination. The tandem cell voltage is equal to the sum of the constituent sub-cells, indicating that the transparent, Ag nanoparticle/MoO3 compound charge recombination layer interposed between the cells is nearly lossless.

Role of transition metal oxides in the charge recombination layer used in tandem organic photovoltaic cells

The mechanism of charge recombination in transition metal oxide-based interconnectors for tandem organic photovoltaic cells is investigated, where the interconnector is composed of an abrupt heterointerface between a Mg-doped 4,7-diphenyl-1,10-phenanthroline (Mg:BPhen) layer and a MoO3 film. Based on the results of the interface energetics determined by ultraviolet photoelectron spectroscopy, as well as the corresponding device characteristics, it is revealed that the MoO3 layer pronouncedly modifies the energy level alignment of the interconnector, which is beneficial for the charge recombination process at the interface between MoO3 and the adjacent donor material for electrons and holes injected from stacked subcells. The incorporation of Mg:BPhen is essential for the conduction of the generated electrons from the bottom subcell into the conduction band of MoO3.

Graphene Intermediate Layer in Tandem Organic Photovoltaic Cells

AbstractOne strategy to harvest wide spectral solar energy is to stack different bandgap materials together in a tandem solar cell. Here, it is demonstrated that CVD grown graphene film can be employed as intermediate layer (IML) in tandem solar cells. Using MoO3‐modified graphene IML, a high open circuit voltage (Voc) of 1 V and a high short‐circuit current density (Jsc) of 11.6 mA cm‐2 could be obtained in series and parallel connection, respectively, in contrast to a Voc of 0.58 V and Jsc of 7.6 mA cm‐2 in single PV cell. The value of Voc (Jsc) in the tandem cell is very close to the sum of Voc (Jsc) attained from two single subcells in series (parallel), which confirms good ohmic contact at the photoactive layer/MoO3‐modified graphene interface. Work function engineering of the graphene IML with metal oxide is essential to ensure good charge collection from both subcells.

Monolithic parallel tandem organic photovoltaic cell with transparent carbon nanotube interlayer

We demonstrate an organic photovoltaic cell with a monolithic tandem structure in parallel connection. Transparent multiwalled carbon nanotube sheets are used as an interlayer anode electrode for this parallel tandem. The characteristics of front and back cells are measured independently. The short circuit current density of the parallel tandem cell is larger than the currents of each individual cell. The wavelength dependence of photocurrent for the parallel tandem cell shows the superposition spectrum of the two spectral sensitivities of the front and back cells. The monolithic three-electrode photovoltaic cell indeed operates as a parallel tandem with improved efficiency.

All-organic tunnel junctions as connecting units in tandem organic solar cell

We employed organic heterojunction films as all-organic connecting units to fabricate tandem organic photovoltaic cells by continuous deposition. The all-organic connecting units with a better transparence and a lower sublimation temperature became an effective recombination center for electrons and holes photogenerated in front cell and back cell, respectively. Tunnel mechanism was proposed to explain the combination of photogenerated carrier. In the optimized tandem solar cell in which tin phthalocyanine dichloride/copper hexadecafluorophthalocyanine heterojunction is used as connecting units, the open-circuit voltage of 1.04 V is double of the single unit cells, and the PCE was increased by 60%, reaching to 1.8%. Therefore, it is an effective way to use the organic heterojunction films as connecting units in organic tandem cells to improve the device performance dramatically.

Simple tandem organic photovoltaic cells for improved energy conversion efficiency

We proposed and demonstrated a simple tandem structure of organic photovoltaic (PV) cell for efficient light harvesting. In this device structure, a soluble fullerene derivative of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is employed simultaneously to form a bilayer heterojunction PV subcell with the underlying copper phthalocyanine (CuPc) and a bulk heterojunction PV subcell with blended poly(3-hexylthiophene-2,5-diyl) (P3HT). In comparison with the conventional tandem structure, the omission of the semitransparent intercellular connection layer reduces the complexity of the device and the light loss. The enhanced short circuit current density (JSC=8.63mA∕cm2) and power conversion efficiency (PCE) (2.79%) of the tandem structure are nearly the sum of those of the stand-alone cells of CuPc/PCBM (JSC=2.09mA∕cm2, PCE=0.43%) and P3HT:PCBM (JSC=6.87mA∕cm2, PCE=2.50%).

ITO-Based Cathode for Application in Semitransparent Organic Photovoltaic Cells

AbstractA high performance semitransparent cathode is one of the major impediments to the high performance semitransparent and tandem organic photovoltaic (OPV) cells. In this work, we discuss the possible designs of semitransparent cathode to improve the performance of semitransparent OPV cells. The optical properties of the poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM)-based OPV cells were studied by the optical admittance analysis. The performance of the OPV cells made with an opaque Ca(10nm)/Ag(100nm) cathode and a semitransparent Ca(10nm)/Ag(10nm)/ITO cathode are discussed. The interfacial properties at the cathode/organic interface were analyzed using the time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The TOF-SIMS depth profile revealed that calcium oxide formed at the Ca/organic interface in semitransparent OPV cells, which was induced by the ITO sputtering process. It shows that the presence of calcium oxide at the organic/cathode interface could be the main reason resulting in a poor fill factor of a semitransparent OPV cell (∼23%) as compared to that of a reference cell (∼43%).

Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions

We demonstrate high-efficiency organic photovoltaic cells by stacking two hybrid planar-mixed molecular heterojunction cells in series. Absorption of incident light is maximized by locating the subcell tuned to absorb long-wavelength light nearest to the transparent anode, and tuning the second subcell closest to the reflecting metal cathode to preferentially absorb short-wavelength solar energy. Using the donor, copper phthalocyanine, and the acceptor, C60, we achieve a maximum power conversion efficiency of ηP=(5.7±0.3)% under 1 sun simulated AM1.5G solar illumination. An open-circuit voltage of VOC⩽1.2V is obtained, doubling that of a single cell. Analytical models suggest that power conversion efficiencies exceeding 6.5% can be obtained by this architecture.

Long-range absorption enhancement in organic tandem thin-film solar cells containing silver nanoclusters

We investigate the optical properties of silver nanoparticles used in tandem ultrathin-film organic photovoltaic cells. Experimental results indicate that the enhancement of an incident optical field persists into an organic dielectric for distances of up to 10nm from the center of an array of approximately 5-nm-diameter nanoparticles. Furthermore, this enhancement exists far from the resonant particle surface-plasmon excitation energy. We propose a model to explain this long-range enhancement and investigate the role that cluster spacing, shape, and an embedding dielectric medium with a complex dielectric constant play in determining plasmon enhancement. This effect is shown to increase the efficiency of tandem organic solar cells, and the implications for further solar cell efficiency improvements are discussed.