Measuring the External Quantum Efficiency of Two‐Terminal Polymer Tandem Solar Cells

Tandem configurations, in which two cells are stacked and connected in series, offer a viable approach to further increase the power conversion efficiency (PCE) of organic solar cells. To enable the future rational design of new materials it is important to accurately assess the contributions of individual subcells. Such accurate measurement of the external quantum efficiency (EQE) of the subcells of two‐terminal organic or polymer tandem solar cells poses specific challenges, caused by two characteristics of these cells, i.e. a sub‐linear light intensity dependence of the current and a field‐assisted charge collection. These properties necessitate that EQE experiments are carried out under representative illumination conditions and electrical bias to maintain short‐circuit conditions for the addressed subcell. We describe a method to determine the magnitudes of the bias illumination and bias voltage during EQE measurements, based on the behavior of single junction cells and optical modeling. The short‐circuit current densities of the subcells obtained by convolution of the EQE with the AM1.5G solar spectrum are consistent with those obtained from optical modeling and correctly predict the current density–voltage characteristics of the tandem cell under AM1.5G conditions.     

Accurate Measurement and Characterization of Organic Solar Cells

Methods to accurately measure the current–voltage characteristics of organic solar cells under standard reporting conditions are presented. Four types of organic test cells and two types of silicon reference cells (unfiltered and with a KG5 color filter) are selected to calculate spectral‐mismatch factors for different test‐cell/reference‐cell combinations. The test devices include both polymer/fullerene‐based bulk‐heterojunction solar cells and small‐molecule‐based heterojunction solar cells. The spectral responsivities of test cells are measured as per American Society for Testing and Materials Standard E1021, and their dependence on light‐bias intensity is reported. The current–voltage curves are measured under 100 mW cm^–2 standard AM 1.5 G (AM: air mass) spectrum (International Electrotechnical Commission 69094‐1) generated from a source set with a reference cell and corrected for spectral error.     

Determination of the optical constants of bulk heterojunction active layers from standard solar cell measurements

The determination of the optical constants n(λ) and k(λ) for organic bulk heterojunction (BHJ) active layers from standard solar cell measurements is presented. We show for a small molecule based as well as for polymer solar cells that the complex refractive index can be derived from the external quantum efficiency (EQE) in combination with current–voltage curves obtained from a series of devices with different active layer thicknesses. The results are compared to those obtained via established techniques and the impact of differences in n(λ) and k(λ) on the solar cells is shown by simulation of the current density using a transfer matrix model.     

Different orientation of the transition dipole moments of two similar Pt(II) complexes and their potential for high efficiency organic light-emitting diodes

The efficiency of organic light-emitting diodes (OLEDs) is especially limited by their low light outcoupling efficiency. An approach for its enhancement is the use of horizontally oriented emitter molecules with respect to the substrate. In this study we quantitatively determine the orientation of the optical transition dipole moments in doped films of two similar phosphorescent Pt(II) complexes having a linear molecular structure. These emitters are employed in OLED devices and their efficiency is analyzed by optical simulations. For an OLED with slightly more horizontally oriented emitter molecules an external quantum efficiency (η_EQE) of 15.8% at low current-density is realized, indicating a relative improvement of outcoupling efficiency of 5.3% compared to the isotropic case. However, a very similar complex adopting isotropic molecular orientation yields η_EQE of only 11.5% implying an imperfect charge carrier balance in the OLED device and a shift of the recombination zone. Furthermore, we highlight the enormous potential of horizontal molecular orientation of emitting molecules in OLEDs.     

Efficient DPP Donor and Nonfullerene Acceptor Organic Solar Cells with High Photon‐to‐Current Ratio and Low Energetic Loss

The high crystallinity and ability to harvest near‐infrared photons make diketopyrrolopyrrole (DPP)‐based polymers one of the most promising donors for high performing organic solar cells (OSCs). However, DPP‐based OSC devices still suffer from the trade‐off between energetic loss (E_loss) and maximum external quantum efficiency (EQE_max), which significantly hinders their potential. Thus far, the replacement of fullerenes with small molecule acceptors did not wisdom the performance development of DPP‐donor‐based solar cells due to severe charge recombination issues. In this work, efficient DPP‐based solar cells are reported using low bandgap fused ring electron acceptor, IEICO‐4F. PBDTT‐DPP:IEICO‐4F OSC devices deliver a champion power conversion efficiency of 9.66% with successful interface engineering along with low E_loss of 0.57 eV and a high EQE_max (>70%).     

Self‐consistent electrical parameter extraction from bias dependent spectral response measurements of III‐V multi‐junction solar cells

Spectral response of multi‐junction solar cell is complicated because of the interplay between external measurement conditions such as bias light intensity, monochromatic light intensity, bias voltage, and intrinsic electrical properties of series interconnected subcells. In this paper, we report an experimental study on the bias voltage‐dependent spectral response (SR) for multi‐junction solar cell. A self‐consistent iteration loop was developed from a nonlinear least square Powell hybrid algorithm that was used for curve fitting the experimental SR versus bias voltage data of each subcell. We demonstrated for the first time that this approach enabled us to derive the electrical parameters such as dark saturation currents (J₀), shunt resistance (R_sh), series resistance (R_s), and spectra response (J_photo) for each subcell of a Ga_0.99In_0.01As/Ge dual junction solar cell with stable convergence. The accuracies of the fitting results were confirmed by the agreement between the J–V curves calculated on the basis of these parameters and the experimental J–V curve of multi‐junction solar cell measured under AM1.5 and 1 sun condition. Copyright © 2013 John Wiley & Sons, Ltd.     

Luminescent cathode buffer layer for enhanced power conversion efficiency and stability of bulk-heterojunction solar cells

We report high photovoltaic efficiency of over 9% in solution-processed, small-molecule (SPSM) 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c]1,2,5]thiadiazole) p-DTS(FBTTh₂)₂:[6-6]-phenyl C70 butyric acid methyl ester (PC₇₀BM) blend based inverted BHJ solar cell by incorporating luminescent zinc oxide doped with sodium (ZnO:Na) quantum dots (QD) (l-ZnO) as a cathode buffer layer (CBL) in inverted bulk-heterojunction (BHJ) solar cells for the first time. The l-ZnO absorbs ultraviolet (UV) light and down-converts it to visible light. The l-ZnO layer's emission overlaps significantly with the absorption of p-DTS(FBTTh₂)₂, leading to an enhanced absorption by p-DTS(FBTTh₂)₂. This resulted in a significant enhancement of photo-current from 15.4 to 17.27 mA/cm² and efficiency from 8% to 9.2% for ZnO and l-ZnO based devices, respectively. This is among one of the highest efficiency values reported so far in the case of SPSM based single junction BHJ solar cells. The luminescent ZnO layer also protects the active layer from UV-induced degradation as solar cells show high stability under constant solar light illumination retaining more than 90% (∼28 h) of its initial efficiency, whereas BHJ solar cells without the luminescent ZnO layer degraded to ∼50% of its initial value under same conditions. Since ZnO is an essential part of inverted organic solar cells, the luminescent l-ZnO CBL has great potential in inverted organic solar cells.     

Evidences of photocurrent generation by hole–exciton interaction at organic semiconductor interfaces

The charge–exciton interaction at the donor/acceptor interface plays a significant role in the exciton dissociation processes, and thus influences the performance of organic solar cells. In this work, the evidences of photocurrent generation via hole–exciton interaction (HEI) at the organic semiconductor interface in organic solar cells, which is the counterpart of photocurrent generated by electron–exciton interaction, is demonstrated. A heterojunction, composed of copper phthalocyanine (CuPc) and fullerene (C₆₀), is used to provide free holes that interact with the excitons supplied by perfluorinated hexadecafluorophthalo-cyaninatozinc (F16ZnPc). The fact that photocurrent generation via HEI is well evidenced by: (1) a short circuit current of 0.38 mA cm⁻²; (2) the jump of an external quantum efficiency (EQE) around 800 nm after adding a bias light; (3) the EQE variations under bias light of different wavelengths and light intensities; and (4) the superlinear dependence of the photocurrent on the light intensity.     

On the application of thin films of silicon nanoparticles for increasing solar cell efficiency

The effect of thin films of silicon nanoparticles (nc-Si), deposited onto the front surface of single-crystal silicon solar cells, on their conversion efficiency is studied. The thin films are grown using non-luminescent silicon nanoparticles with an average diameter of 12 nm with SiO _x (0 ≤ x ≤ 2) shells and silicon nanoparticles 2 nm in diameter with organic shells of octadecene, which exhibit photoluminescence in the red spectral region. It was found that nc-Si film deposition increases the solar-cell conversion efficiency by 12% with respect to the initial value. An analysis of the current-voltage characteristics and reflectance spectra of solar cells allows the conclusion that the increase in the conversion efficiency is controlled by the passivation of defects on the front surface of the solar cell by nanoparticles and a decrease in the light reflectance of this surface.     

Spectral mismatch correction for GaAs solar cells with varying junction depths

This study experimentally demonstrates that large errors 25%) in solar cell current measurements can occur even if a properly calibrated reference cell of the same material system is used to set the intensity of the light source. The spectral mismatch correction procedure allows the accurate (2%) measurement of the illuminated I-V characteristics. This is done with respect to standard test conditions defined by the temperature, intensity and reference spectrum. The results are independent of the light source spectral irradiance, or reference cell spectral response.     

Effect of illumination wavelength and intensity on minority-carrier diffusion length of EFG silicon ribbon solar cells

Local spectral photoresponses of the edge-defined film-fed growth technique (EFG) ribbon solar cells were measured as a function of the bias light wavelength and intensity. The minority-carrier diffusion length estimated from the spectral response measurement of the short-circuit current (JSc) was found to increase semilogarithmically with the intensity at various bias light wavelengths, and to increase with the bias fight wavelength for the constant intensity in the position with a medium diffusion length (≃20 µm). In the position with a longer (≃89 µm) or shorter (≃7 µm) diffusion length, the bias light wavelength, however, had a smaller influence on the diffusion length. The effect of the bias light wavelength and intensity on the diffusion length is theoretically explained by a model of filling deep traps by photo-injected minority carriers.     

Thermal analysis of organic solar cells using an enhanced opto-thermal model

In this paper, an opto-thermal model is presented in order to specify the dominant thermal phenomena in organic solar cells (OSCs), as rather low efficiency photovoltaic devices. This model is capable of predicting the amount of optical heat generation (Q_{th_opt}), also the transient and steady state thermal behavior of an organic photovoltaic cell combining both the optical and thermal models. In a typical organic solar cell, Q_{th_opt} plays a significant role in heating up the device while the electric heat generation (Q_{th_elec}) does not effectively have such a role. Developing an optical model for a solar cell, Q_{th_opt} can be determined in every position of the device; also, the contribution of each layer in heat generation is precisely specified. The device thermal behavior is predicted by feeding the thermal model with Q_{th_opt}. This is done for an organic solar cell with a typical architecture and it is shown that thermal convection and radiation are two determinative thermal phenomena while conduction plays a minor role; furthermore, the electrodes, Aluminum (Al) cathode and Indium Tin Oxide (ITO) anode, are two strong light absorbers which contribute to more than 80% of optical heat generation. Assuming Stefan–Boltzman radiation loss, the temperature rise for a typical single junction OSC is estimated under different conditions. The device temperature rise might be even larger for other architectures consisting of several layers depending on their thicknesses and absorption coefficients. This temperature increase enhances the OSCs’ efficiency while degrading the lifetime. The model can be applied to thermal analysis of other types of photovoltaic cells and optoelectronic devices with minor modification.     

The Relation Between Open‐Circuit Voltage and the Onset of Photocurrent Generation by Charge‐Transfer Absorption in Polymer : Fullerene Bulk Heterojunction Solar Cells

Photocurrent generation by charge‐transfer (CT) absorption is detected in a range of conjugated polymer–[6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (EQE) spectrum by means of Fourier‐transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak ground‐state charge‐transfer complexes in the studied polymer–fullerene blends. The effective band gap (E_g) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all devices, under various preparation conditions, the open‐circuit voltage (V_oc) scales linearly with E_g. The redshift of the CT band upon thermal annealing of regioregular poly(3‐hexylthiophene):PCBM and thermal aging of poly(phenylenevinylene)(PPV):PCBM photovoltaic devices correlates with the observed drop in open‐circuit voltage of high‐temperature treated versus untreated devices. Increasing the weight fraction of PCBM also results in a redshift of E_g, proportional with the observed changes in V_oc for different PPV:PCBM ratios. As E_g corresponds with the effective bandgap of the material blends, a measurement of the EQE spectrum by FTPS allows us to measure this energy directly on photovoltaic devices, and makes it a valuable technique in the study of organic bulk heterojunction solar cells.     

Recombination properties of a diffused pn junction determined by spectral response measurements

The theoretical spectral responsivity of a diffused pn junction is computed in the case of a silicon n⁺p junction which employes a rather deep (4, 7 μ) and lightly doped N⁺ front region.Comparing experimental results with theoretical predictions the diffusion length L and surface recombination velocity S₀ can be determined. Several cases are examined: the influence of an oxide layer on the front and of gettering processes on L and S₀ are presented and the overall sensitivity of the method is discussed.     

Analysis of exciton annihilation in high-efficiency sky-blue organic light-emitting diodes with thermally activated delayed fluorescence

We study external quantum efficiency (η_EQE) roll-off in organic light-emitting diodes (OLEDs) using thermally-activated delayed fluorescence (TADF) of 4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN). Using 2CzPN intramolecular rate constants from optical analyses, we construct an exciton quenching model incorporating intersystem crossing and reverse intersystem crossing. The model indicates that singlet–triplet annihilation and triplet–triplet annihilation dominate η_EQE roll-off because of the relatively long 2CzPN triplet lifetime of 273 μs. This work yields a method to relax the exciton quenching process in TADF based OLEDs.     

Triptycene-derived TADF enantiomers displaying circularly polarized luminescence and high-efficiency electroluminescence

A pair of novel circularly polarized thermally activated delayed fluorescence (CP-TADF) enantiomers (+)-(S,S)-CTRI-Cz and (−)-(R,R)-CTRI-Cz based on chiral triptycene scaffold were designed and synthesized. The obtained triptycene-derived enantiomers displayed obvious TADF activities with small singlet-triplet energy gap value (ΔE_ST) of 0.20 eV and characteristic microsecond delayed lifetime of 15.4 μs. Moreover, the TADF enantiomers showed mirror-image circular dichroism (CD) and circularly polarized luminescence (CPL) activities, and their luminescence dissymmetry factors (g_lum) were about ±0.9 × 10⁻³. Finally, by using the TADF enantiomers as emitters, the optimized organic light-emitting diodes (OLEDs) achieved maximum external quantum efficiency (EQE_max), current efficiency (CE_max) and power efficiency (PE_max) of 15.0%, 48.8 cd/A and 46.9 lm/W, respectively.     

Photovoltaic properties and anomalous effects in the ZnSe-GaAs heterojunction

The photoelectric properties of the nZnSe-pGaAs heterojunction obtained by deposition of ZnSe on GaAs in the MOCVD process have been investigated. The current-voltage characteristics of the illuminated solar cells are shifted to lower voltages than those expected from the superposition principle. The dependence of quantum efficiency and C–V curves on light intensity and wavelength has been observed. A heterojunction model is proposed for both light and dark conditions. We assumed that near the interface a compensated ZnSe layer exists with negative charged deep centers. As a result electric current in the heterojunction is limited by a barrier in the conduction band. Variation of charge in the compensated layer is connected with electron transfer from deep centers to the conduction band. This process explains the anomalous effects for such a heterojunction. The characteristics calculated by this model are close to the experimental curves.     

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.     

Limiting factors of photon-to-current conversion in polymer/nanocrystal bilayer hybrid solar cells: An analytical quantum efficiency model study

This paper introduces an analytical external quantum efficiency (EQE) model of planar hybrid solar cells (HSCs) based on photon-to-current conversion processes and uses this to investigate the factors that limit the maximum EQE (EQE_m) of devices; i.e., the photon absorption coefficient α, exciton diffusion coefficient D_z, exciton lifetime τ_z, exciton dissociation rate k_dis, electron diffusion coefficient D_e, electron lifetime τ_e, nanocrystals thickness d, and thickness of the polymer l. Our simulations indicate that relying solely on modifying k_dis, D_e, or τ_e cannot achieve a breakthrough increase in the EQE_m of planar HSCs. However, increasing α, D_z, or τ_z could potentially lead to a large EQE_m (30–100%), especially in the context of high k_dis values. Moreover, the calculation results indicate that although both D_z and τ_z contribute to the exciton diffusion length (L_z) via the equation L_z² = D_zτ_z, the EQE_m has an asymmetric dependence on these variables. With a small k_dis (i.e., <10⁴ cm/s), an increase in D_z results in an initial increase and then decrease in EQE_m, resulting in a peak value that increases with increasing k_dis. When k_dis is sufficiently large (∼10⁵ cm/s), the EQE_m becomes saturated after the initial increase. Thus, although an increase in D_z can adversely affect device performance when the k_dis is lower than 10⁴ cm/s, increasing τ_z always improves device performance, regardless of large k_dis becomes. This behavior can be attributed to the detrimental effect of excitons accumulating at the D/A interface, and can be used to optimize the material design and device engineering of planar HSCs and related solar cells for maximum photon-to-current conversion performance. In addition, we also demonstrate that the model can fit to the experimental data.     

Injection photodiode based on an n-CdS/p-CdTe heterostructure

The possibility of developing injection photodiodes with a tunable/reconfigurable? photosensitivity spectrum in the spectral range of 500–800 nm based on an n-CdS/p-CdTe heterostructure is shown. It is established that such a structure in the short-wavelength region λ = 500 nm has the highest spectral sensitivity S _λ ≈ 3 A/W in the forward direction at a bias voltage of V = +120 mV and S _λ ≈ 2 A/W in the reverse direction at a bias voltage of V = ?120 mV. The integrated sensitivity of the device is S _int = 2 400 A/lm under illumination with white light E = 3 × 10^?2 lx, at a bias voltage of V = +4.6 V, and temperature of T = 293 K. Upon illumination with the monochromatic light of an LG-75 laser with the wavelength λ = 625 nm, S _int = ?1400 A/W (illumination power P = 18 × 10^?6 W/cm², bias voltage V = +4.6 V, and temperature T = 293 K). High values of S _λ and S _int provide the highly efficient transformation of light energy into electrical energy at low illumination levels (P < 18 × 10^?6 W/cm²).