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.     

Synthesis and characterization of the conjugated polymers tethered with dipolar side chains containing a benzothiadiazole entity for bulk heterojunction solar cells

New conjugated copolymers (P1?P3) containing dipolar side chains connected to the main chain via triphenylamine donors have been synthesized and characterized. The side chains of these polymers have an electron deficient benzothiadiazole moiety in the spacer, but with different acceptors at the end. By changing the acceptor moieties of the side chain, the absorption spectra and HOMO/LUMO gaps of the polymers can be fine-tuned, ranging from 1.86 to 1.59 eV. Solution processed bulk heterojunction (BHJ) solar cells using these polymers as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor were fabricated and measured under 100 mW cm^?2 of AM 1.5 illumination. The cell based on the blend of P1/PCBM (1:1, w/w) exhibited the highest power conversion efficiency of 1.78%, with open circuit voltage (V_oc) = 0.79 V, short circuit current (J_sc) = 6.63 mA cm^?2 and fill factor (FF) = 0.34, respectively.     

High‐speed atmospheric atomic layer deposition of ultra thin amorphous TiO2 blocking layers at 100 °C for inverted bulk heterojunction solar cells

Ultrafast, spatial atmospheric atomic layer deposition, which does not involve vacuum steps and is compatible with roll‐to‐roll processing, is used to grow high quality TiO₂ blocking layers for organic solar cells. Dense, uniform thin TiO₂ films are grown at temperatures as low as 100 °C in only 37 s (~20 nm/min growth rate). Incorporation of these films in P3HT‐PCBM‐based solar cells shows performances comparable with cells made using TiO₂ films deposited with much longer processing times and/or higher temperatures. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of molar mass ratio,annealing temperature and cathode buffer layer on power conversion efficiency of P3HT:PC71BM based organic bulk heterojunction solar cell

In bulk heterojunction (BHJ) solar cells, the molar mass ratio of donor-acceptor polymers, the annealing temperature (T_an) and the cathode buffer layer plays very consequential role in improving the power conversion efficiency (PCE) by tuning the film morphology and enhancing the charge carrier dynamics. A comprehensive understanding of each of these factors is essential in order to optimize the performance of organic solar cells (OSCs). Albeit there are several fundamental reports regarding these factors, an altogether meticulous correlation of these physical processes with experimental evidence of the photo active layer are required. In this work, we systematically analyzed the influence of different molar mass ratio, the annealing temperature (T_an) and the cathode buffer layer of rrP3HT:PC₇₁BM based BHJ solar cells and their corresponding photovoltaic performances were correlated carefully with their thin film growth structure and energy level diagram. The device having 1:0.8 molar mass ratio of rrP3HT:PC₇₁BM and T_an = 150 °C annealing temperature with Bathocuproine (BCP) as the cathode buffer layer having ITO/PEDOT:PSS/rrP3HT:PC₇₁BM (molar mass ratio = 1:0.8; (T_an = 150 °C)/BCP/Al) configuration showed the best device performance with PCE, ɳ = 4.79%, Jsc = 14.21 mA/cm², V_oc = 0.58 V and FF = 57.8%. This drastic variation in PCE of the device having BCP/Al as the cathode contact compared to the other device configurations is due to the coalesced effects of better hole-blocking capacity of BCP along with Al and better phase separation of the active blend layer at 150 °C annealing temperature. These results explicate the cumulate role of all these physical parameters and their combined contribution to the PCE amendment and overall device performance with rrP3HT:PC₇₁BM based organic BHJ solar cell.     

Characterization of polymer bulk heterojunction photocell with unmodified C70 prepared with halogen-free solvent for indoor light harvesting

Although photocells are commonly characterized under AM1.5G 100 mW cm⁻² (1 sun) illumination, their performance under low light illumination is also important, because photocells are frequently used for indoor applications. In this study, polymer photocells based on a bulk heterojunction composite consisting of a low energy gap polymer PTB7 and unmodified C₇₀ prepared with a halogen-free solvent 1,2,4-trimethylbenzene have been characterized under the illumination of 1 sun or below. A typical photocell with the power conversion efficiency (PCE) of 4% at 1 sun shows the PCE of approximately 7% at 10⁻³ sun, which seems to fit for some indoor applications such as a permanent power source for a wireless sensor node. The sublinear dependence of short-circuit photocurrent on light intensity as well as the increase of fill-factor under low light illumination yields the increased efficiency under low light illumination. An analysis employing a one-diode equivalent circuit model suggests that the increased parallel resistance as well as the decreased saturation current of the diode under low light illumination accounts for the latter feature. It is also pointed out that the parallel resistance and/or the saturation current under dark strongly influence the PCE of a photocell under low light illumination. In addition, the dependence of the device performance on the light intensity is found to be useful for analyzing the effects of the thermal treatment and the PFN interlayer at cathode.     

Synthesis and characterization of a new perylene bisimide (PBI) derivative and its application as electron acceptor for bulk heterojunction polymer solar cells

A symmetrical perylene bisimide derivative (PBI) with 2-(4-nitrophenyl)acrylonitrile groups at the 1,7 bay positions of perylene and solubilizing cyclohexyl units was synthesized and characterized. The absorption spectrum of PBI was broad with the most prominent peak at 655 nm and optical band gap of 1.72 eV. The electrochemical investigation indicates that PBI has a LUMO energy level of −3.9 eV which is similar to that of PCBM or PC₇₀BM. Bulk heterojunction solar cell fabricated using a blend of poly(3-hexylthiophene) (P3HT) and PBI (1:1 w/w) as active layer cast from THF exhibited power conversion efficiency (PCE) at 1.56%. However, the device with P3HT:PBI blend deposited from mixed solvent (DIO/THF) improved the PCE to 2.78% which further increased to 3.17% on using the thermal annealed active layer. The improvement in the PCE is attributed to the enhanced crystallinity of the blend (particularly P3HT) and increase in hole mobility leading to balanced charge transport.     

A comparative study of metamorphic InP/InGaAs double heterojunction bipolar transistors with InP and InAIP buffer layers grown by molecular beam epitaxy

We present a comparison of material quality and device performance of metamorphic InGaAs/InP heterojunction bipolar transistors (HBTs) grown by molecular beam epitaxy (MBE) on GaAs substrates with two different types of buffer layers (direct InP and graded InAlP buffers). The results show that the active layer of InP-MHBT has more than one order of magnitude more defects than that of the InAlP-MHBT. The InAlP-MHBTs show excellent direct current (DC) performance. Low DC current gain and a high base junction ideality factor from the InP-MHBT are possibly due to a large number of electrically active dislocations in the HBT active layers, which is consistent with a large number of defects observed by cross-sectional transmission electron microscopy (TEM) and rough surface morphology observed by atomic force microscopy (AFM).     

Effects of the incorporation of an additional pyrrolo[3,4-c]pyrrole-1,3-dione unit on the repeating unit of highly efficient large band gap polymers containing benzodithiophene and pyrrolo[3,4-c]pyrrole-1,3-dione derivatives

Property modulation of 2,5-dioctylpyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (DPPD)-based high energy converting wide band gap polymers, P(BDT-TDPPDT) and P(BDTT-TDPPDT), was studied via the incorporation of an additional DPPD unit on their repeating units. A new electron acceptor (BDPPD) unit containing two DPPD units was prepared and copolymerized with the distannyl derivatives of benzodithiophene (BDT) or 2D-conjugated benzodithiophene (BDTT) to provide two new polymers, P(BDT-BDPPD) and P(BDTT-BDPPD). The optical band gaps and highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels of P(BDT-BDPPD) and P(BDTT-BDPPD) were 2.16 eV, 2.08 eV and −5.37 eV/‒3.21 eV, −5.44 eV/‒3.36 eV, respectively. The hole mobilities of P(BDT-BDPPD) and P(BDTT-BDPPD) were in the order of 10⁻⁴ cm²V⁻¹ s⁻¹. The polymer solar cells (PSCs) prepared with the configuration of ITO/PEDOT:PSS/P(BDT-BDPPD) or P(BDTT-BDPPD):PC₇₀BM/Al gave maximum power conversion efficiencies (PCEs) of 2.74% and 3.63%, respectively. The insertion of a BDPPD unit instead of a TDPPDT unit on the DPPD-based polymer backbone did not affect the optical and electrochemical properties considerably. On the other hand, the new polymers, P(BDT-BDPPD) and P(BDTT-BDPPD), resulted in improved photovoltaic performances compared to the reported polymers, P(BDT-TDPPDT) and P(BDTT-TDPPDT), for the devices prepared without additives.     

Evaluation of the influence of an embedded porous silicon layer on the bulk lifetime of epitaxial layers and the interface recombination at the epitaxial layer/porous silicon interface

Porous silicon plays an important role in the concept of wafer‐equivalent epitaxial thin‐film solar cells. Although porous silicon is beneficial in terms of long‐wavelength optical confinement and gettering of metals, it could adversely affect the quality of the epitaxial silicon layer grown on top of it by introducing additional crystal defects such as stacking faults and dislocations. Furthermore, the epitaxial layer/porous silicon interface is highly recombinative because it has a large internal surface area that is not accessible for passivation. In this work, photoluminescence is used to extract the bulk lifetime of boron‐doped (10¹⁶/cm³) epitaxial layers grown on reorganised porous silicon as well as on pristine mono‐crystalline, Czochralski, p⁺ silicon. Surprisingly, the bulk lifetime of epitaxial layers on top of reorganised porous silicon is found to be higher (~100–115 µs) than that of layers on top of bare p⁺ substrate (32–50 µs). It is believed that proper surface closure prior to epitaxial growth and metal gettering effects of porous silicon play a role in ensuring a higher lifetime. Furthermore, the epitaxial layer/porous silicon interface was found to be ~250 times more recombinative than an epitaxial layer/p⁺ substrate interface (S ≅ 10³ cm/s). However, the inclusion of an epitaxially grown back surface field on top of the porous silicon effectively shields minority carriers from this highly recombinative interface. Copyright © 2013 John Wiley & Sons, Ltd.