Understanding the correlation between energy-state mismatching and open-circuit voltage loss in bulk heterojunction solar cells

Photoinduced intermolecular charge transfer(PICT)determines the voltage loss in bulk heterojunction(BHJ)organic photovoltaics(OPVs),and this voltage loss can be minimized by inducing efficient PICT,which requires energy-state matching between the donor and acceptor at the BHJ interfaces.Thus,both geometrically and energetically accessible delocalized state matching at the hot energy level is crucial for achieving efficient PICT.In this study,an effective method for quantifying the hot state matching of OPVs was developed.The degree of energy-state matching between the electron donor and acceptor at BHJ interfaces was quantified using a mismatching factor(MF)calculated from the modified optical density of the BHJ.Furthermore,the correlation between the open-circuit voltage(Voc)of the OPV device and energy-state matching at the BHJ interface was investigated using the calculated MF.The OPVs with small absolute MF values exhibited high Voc values.This result clearly indicates that the energy-state matching between the donor and acceptor is crucial for achieving a high Voc in OPVs.Because the MF indicates the degree of energy-state matching,which is a critical factor for suppressing energy loss,it can be used to estimate the Voc loss in OPVs.

Influence of donor:acceptor ratio on charge transfer dynamics in non-fullerene organic bulk heterojunctions

The donor:acceptor(D:A) blend ratio plays a very important role in affecting the progress of charge transfer and energy transfer in bulk heterojunction(BHJ) orga nic solar cells(OSCs).The proper D:A blend ratio can provide maximized D/A interfacial area for exciton dissociation and appro p riate domain size of the exciton diffusion length,which is beneficial to obtain high-performance OSCs.Here,we comprehensively investigated the relationship between various D:A blend ratios and the charge transfer and energy transfer mechanisms in OSCs based on PBDB-T and non-fullerene acceptor IT-M.Based on various D:A blend ratios,it was found that the ratio of components is a key factor to suppress the formation of triplet states and recombination energy losses.Rational D:A blend ratios can provide appropriate donor/accepter surface for charge transfer which has been powerfully verified by various detailed experimental results from the time-resolved fluorescence measurement and transient absorption(TA) spectroscopy.Optimized coherence length and crystallinity are verified by grazing incident wide-angle X-ray scattering(GIWAXS) measurements.The results are bene ficial to comprehend the effects of various D:A blend ratios on charge transfer and energy transfer dynamics and provides constructive suggestions for rationally designing new materials and feedback for photovoltaic performance optimization in non-fullerene OSCs.

Probing molecular orientation at bulk heterojunctions by polarization-selective transient absorption spectroscopy

A bulk heterojunction in organic solar cells is where charge separation and recombination occur.Molecular orientation at the interface is one of the key factors that dictate solar cell efficiency.Although X-ray scattering-based methods can determine donor/acceptor domain orientations between an anisotropic phase and an isotropic fullerene-based phase,the rise of nonfullerene solar cells presents a new challenge in delineating local molecular directions at the interface between two anisotropic donor/acceptor domains.Here,we determine interfacial molecular orientations of three high-efficiency small molecule solar cells(ZR1:Y6,B1:BO-4 Cl,and BTR:BO-4 Cl)using polarization-selective transient absorption spectroscopy.The polarization anisotropy of charge separation dynamics indicates an angle of~90°between ZR1 and Y6 molecules at the interface,an angle close to 0°between B1 and BO-4 Cl,and random orientations between BTR and BO-4 Cl.These observations provide complementary information to X-ray scattering measurements and highlight polarization-selective transient absorption spectroscopy as a tool to probe interfacial structure and dynamics of key photophysical steps in energy conversion.

Enhanced charge separation by interchain hole delocalization in nonfullerene acceptor-based bulk heterojunction materials

Bulk heterojunction(BHJ)composites show improved power conversion efficiencies when optimized in terms of morphology using various film processing methods.A reduced carrier recombination loss in an optimized BHJ was characterized previously.However,the driving force that leads to this reduction was not clearly understood.In this study,we focus on the decreased carrier recombination loss and its driving force in optimized nonfullerene acceptor-based PTB7-Th:IEICO-4F BHJ composites.We demonstrate that the optimized BHJ shows deactivation in the sub-nanosecond nongeminate carrier recombination process.The driving force for this deactivation was determined to be the improved interchain hole delocalization between the polymers.An enhanced interchain hole delocalization was observed using steady-state photoinduced absorption(PIA)spectroscopy.In particular,increased splitting between the polaron PIA bands was noted.Moreover,improved interchain hole delocalization was observed for other state-of-the-art BHJ materials,including D18:Y6 with optimized morphologies.

Effect of Valence Band Tail Width on the Open Circuit Voltage of P3HT:PCBM Bulk Heterojunction Solar Cell:AMPS-1D Simulation Study

The effect of the valence band tail width on the open circuit voltage of P3HT：PCBM bulk heterojunction solar cell is investigated by using the AMPS-1D computer program. An effective medium model with exponential valence and conduction band tail states is used to simulate the photovoltaic cell. The simulation result shows that the open circuit voltage depends Iinearly on the logarithm of the generation rate and the slope depends on the width of the valence band tail. The open circuit voltage decreases with the increasing width of the band tail. The dark and light ideality factors increase with the width of the valence band tail.

Simulation study of the losses and influences of geminate and bimolecular recombination on the performances of bulk heterojunction organic solar cells

We use the method of device simulation to study the losses and influences of geminate and bimolecular recombinations on the performances and properties of the bulk heterojunction organic solar cells. We find that a fraction of electrons（holes）in the device are collected by anode（cathode）. The direction of the corresponding current is opposite to the direction of photocurrent. And the current density increases with the bias increasing but decreases as bimolecular recombination（BR）or geminate recombination（GR） intensity increases. The maximum power, short circuit current, and fill factor display a stronger dependence on GR than on BR. While the influences of GR and BR on open circuit voltage are about the same.Our studies shed a new light on the loss mechanism and may provide a new way of improving the efficiency of bulk heterojunction organic solar cells.

MEH-PPV/Alq_3-based bulk heterojunction photodetector

In this paper, we present the effect of varied illumination levels on the electrical properties of the organic blend bulk heterojuction （BHJ） photodiode. To prepare the BHJ blend, poly（2-methoxy-5（2P-ethylhexyloxy） phenyleneviny- lene （MEH-PPV） and aluminum-tris-（8-hydroxyquinoline） （Alq3） are used as donor and acceptor materials, respectively. In order to fabricate the photodiode, a 40-nm thick film of poly（3, 4-ethylendioxytbiophene）：poly（styrensulfonate） （PE- DOT：PSS） is primarily deposited on a cleaned ITO coated glass substrate by spin coating technique. The organic photo- sensitive blend is later spun coated on the PEDOT：PSS layer, followed by the lithium fluoride （LiF） and aluminium （A1） thin films deposition by thermal evaporation. The optical properties of the MEH-PPV：Alq3 blend thin films are investigated using photoluminescence （PL） and UV-Vis spectroscopy. The photodiode shows good photo-current response as a function of variable illumination levels. The responsivity value - 8 mA/W at 3 V is found and the ratio of photo-current to dark current （lph/IDark） is found to be 1.24.

Study of a ternary blend system for bulk heterojunction thin film solar cells

In this research, we report a bulk heterojunction（BHJ） solar cell consisting of a ternary blend system. Poly（3-hexylthiophene） P3 HT is used as a donor and [6,6]-phenyl C61-butyric acid methylester（PCBM） plays the role of acceptor whereas vanadyl 2,9,16,23-tetraphenoxy-29 H, 31H-phthalocyanine（VOPc Ph O） is selected as an ambipolar transport material. The materials are selected and assembled in such a fashion that the generated charge carriers could efficiently be transported rightwards within the blend. The organic BHJ solar cells consist of ITO/PEDOT：PSS/ternary BHJ blend/Al structure. The power conversion efficiencies of the ITO/ PEDOT：PSS/P3HT：PCBM/Al and ITO/PEDOT：PSS/P3HT：PCBM：VOPcPhO/Al solar cells are found to be 2.3% and 3.4%, respectively.

Effect of Crystallinity of Fullerene Derivatives on Doping Density in the Organic Bulk Heterojunction Layer in Polymer Solar Cells

Polymer solar cells （PSCs） based on poly（3-hexylthiophene） （P3HT） and [6,6]-phenyl-C61-butyric acid methyl ester （PCBM） are fabricated by using 1,8-diiodooctane （DIO） as a solvent additive to control the doping density of the PSCs. It is shown that the processing of DIO does not change the doping density of the P3HT phase, while it causes a dramatic reduction of the doping density of the PCBM phase, which decreases the doping density of the whole blend layer from 3.7 × 10^16 cm-3 to 1.2 ×10^16 cm-3. The reduction of the doping density in the PCBM phase originates from the increasing crystallinity of PCBM with DIO addition, and it leads to a decreasing doping density in the blend film and improves the short circuit current of the PSCs.

High-performance bulk heterojunction-based photocathode with facile architecture for photoelectrochemical water splitting

Organic semiconductors are promising candidates as photoactive layers for photoelectrodes used in photoelectrochemical(PEC)cells due to their excellent light absorption and efficient charge transport properties with the help of interfacial materials.However,the use of multilayers will make the charge transfer mechanism more complicated and decrease the PEC performance of the photoelectrode caused by the increased contact resistance.In this work,a PM6:Y6 bulk heterojunction(BHJ)-based photocathode is fabricated for efficient PEC hydrogen evolution reaction(HER)in an acidic aqueous solution.With RuO_(2)as an interfacial modification layer,the photocathode with a simple structure(fluorine-doped tin oxide(FTO)/PM6:Y6/RuO_(2))generates a maximum photocurrent density up to-15 m A/cm^(2)at 0 V vs.reference hydrogen electrode(RHE),outperforming all previously reported BHJ-based photocathodes in terms of PEC performance.The highest ratiometric power-saved efficiency of 3.7%is achieved at 0.4 V vs.RHE.

Shorter alkyl chain in thieno[3,4-c]pyrrole-4,6-dione(TPD)-based large bandgap polymer donors – Yield efficient non-fullerene polymer solar cells

Typically,conjugated polymers are composed of conjugated backbones and alkyl side chains.In this contribution,a cost-effective strategy of tailoring the length of alkyl side chain is utilized to design highperforming thieno[3,4-c]pyrrole-4,6-dione(TPD)-based large bandgap polymer donors PBDT-BiTPD(Cχ)(χ=48,52,56),in which x represents the alkyl side chain length in term of the total carbon number.A combination of light absorption,device,and morphology examinations make clear that the shorter alkyl side chains yield(i) higher crystallinity and more predominant face-on crystallite orientation in their neat and BHJ blend films,(ii) higher charge mobilities(6.7×10^(-4) cm~2 V^(-1) s^(-1) for C48 vs.3.2×10^(-4) cm~2 V^(-1) s^(-1) for C56),and negligible charge recombination,consequently,(iii) significantly improved fill-factor(FF) and short current(J_(SC)),while almost the same open circuit voltage(V_(OC)) of ca.0.82 V in their corresponding BHJ devices.In parallel,as alkyl side chain lengths decrease from C56 to C48,power conversion efficiencies(PCEs) increased from 7.8% for C56 to 11.1% for C52,and further to14.1% for C48 in their BHJ solar cells made with a narrow bandgap non-fullerene acceptor Y6.This systematic study declares that shortening the side chain,if providing appropriate solubility in device solution processing solvents,is of essential significance for developing high-performing polymer donors and further improving device photovoltaic performance.

Non-fullerene small molecule electron acceptors for high-performance organic solar cells

Fullerenes and their derivatives are important types of electron acceptor materials and play a vital role in organic solar cell devices. However, the fullerene acceptor material has some difficulties to overcome the intrinsic shortcomings, such as weak absorption in the visible range, difficulty in modification and high cost, which limit the performance of the device and the large-scale application of this type of acceptors. In recent years, non-fullerene electron acceptor material has attracted the attention of scientists due to the advantages of adjustable energy level, wide absorption, simple synthesis, low processing cost and good solubility. Researchers can use the rich chemical means to design and synthesize organic small molecules and their oligomers with specific aggregation morphology and excellent optoelectronic prop- erties. Great advances in the field of synthesis, device engineering, and device physics of non-fullerene acceptors have been achieved in the last few years. At present, non-fullerene small molecules based photovoltaic devices achieve the highest efficiency more than 13% and the efficiency gap between fullerenetype and non-fullerene-type photovoltaic devices is gradually narrowing. In this review, we explore recent progress of non-fullerene small molecule electron acceptors that have been developed and led to highefficiency photovoltaic devices and put forward the prospect of development in the future.

Tandem white organic light-emitting diodes adopting a C_(60) :rubrene charge generation layer

Organic bulk heterojunction fullerence(C60) doped 5, 6, 11, 12-tetraphenylnaphthacene(rubrene) as the high quality charge generation layer(CGL) with high transparency and superior charge generating capability for tandem organic light emitting diodes(OLEDs) is developed. This CGL shows excellent optical transparency about 90%, which can reduce the optical interference effect formed in tandem OLEDs. There is a stable white light emission including 468 nm and 500 nm peaks from the blue emitting layer and 620 nm peak from the red emitting layer in tandem white OLEDs. A high efficiency of about 17.4 cd/A and CIE coordinates of(0.40, 0.35) at 100 cd/m2 and(0.36, 0.34) at 1000 cd/m2 have been demonstrated by employing the developed CGL, respectively.

Optimization of solvent swelling for efficient organic solar cells via sequential deposition

Compared to bulk heterojunction(BHJ)organic solar cells(OSCs)prepared by the blend casting in“one step process”,sequential deposition(SD)processed OSCs can realize an ideal profile of vertical component distribution due to the swelling of polymer films.Herein,we did trials on several kinds of second solvents for swelling the polymer layer,and investigated the packing structure and morphology of the swollen films and the performance of the resulting devices.We found that an optimized morphology can be achieved by solvent swelling while using orthodichlorobenzene(o-DCB)as the second layer processing-solvent,with polymer donor PffBT-3 as bottom layer,PC71BM as top layer and bicontinuous networks in the middle.Such solvent swelling process also makes the SD method exempt from thermal annealing treatment.The device based on SD yields a power conversion effi-ciency(PCE)up to 8.7%without any post-treatment,outperforming those from the devices based on SD using other solvents and that(7.06%)from BHJ device,respectively.We also extended the use of this approach to allpolymer blend system,and successfully improved the efficiency from 4.72%(chloroform)to 9.35%(o-DCB),which is among the highest PCEs in all-polymer-based OSCs fabricated with SD method.The results demonstrate that the swelling of the polymer by the second layer solvent is a necessity for SD,paving the way towards additivefree high-performance OSCs.

Influence of small-molecule material on performance of polymer solar cells based on MEH-PPV:PCBM blend

In this work, the influence of a small-molecule material, tris（8-hydroxyquinoline） aluminum （Alq3）, on bulk heterojunction （BHJ） polymer solar cells （PSCs） is investigated in devices based on the blend of poly（2-methoxy-5-（2- ethylhexyloxy）-1,4-phenylenevinylene） （MEH-PPV） and [6,6]-phenyl-C61-butyric acid methyl ester （PCBM）. By doping Alq3 into MEH-PPV：PCBM solution, the number of MEH-PPV excitons can be effectively increased due to the energy transfer from Alq3 to MEH-PPV, which probably induces the increase of photocurrent generated by excitons dissociation. However, the low carrier mobility of Alq3 is detrimental to the efficient charge transport, thereby blocking the charge collection by the respective electrodes. The balance between photon absorption and charge transport in the active layer plays a key role in the performance of PSCs. For the case of 5 wt.% Alq3 doping, the device performance is deteriorated rather than improved as compared with that of the undoped device. On the other hand, we adopt Alq3 as a buffer layer instead of commonly used LiF. All the photovoltaic parameters are improved, yielding an 80% increase in power conversion efficiency （PCE） at the optimum thickness （1 nm） as compared with that of the device without any buffer layer. Even for the 5 wt.% Alq3 doped device, the PCE has a slight enhancement compared with that of the standard device after modification with 1 nm （or 2 nm） thermally evaporated Alq3. The performance deterioration of Alq3-doped devices can be explained by the low solubility of Alq3, which probably deteriorates the bicontinuous D-A network morphology; while the performance improvement of the devices with Alq3 as a buffer layer is attributed to the increased light harvesting, as well as blocking the hole leakage from MEH-PPV to the aluminum （Al） electrode due to the lower highest occupied molecular orbital （HOMO） level of Alq3 compared with that of MEH-PPV.

Patterning of Poly（3,4-Ethylenedioxythiophene）： Poly（Styrenesulfonate） Films via the Rubbing Method in Organic Photovoltaic Cells

OPV （Organic photovoltaic） cells represent a compelling candidate for renewable energy by solar energy conversion. In recent years, versatile light-trapping measures via structures have been intensively explored to optimize photovoltaic performance. In this work, a unique rubbing technique is demonstrated to create nanoscale grooves on the PEDOT：PSS [poly（3,4-ethylenedioxythiophene）： poly（styrenesulfonate）] surface and the grating-like features are 500 nm wide and 10 nm deep. The PEDOT：PSS film with grooved surface is used as buffer layers for OPV cell devices based on a P3HT：PCBM bulk heterojunction. The patterned surface has a profound effect on carrier mobility, light trapping, and hole collection efficiency, leading to an increase in the short circuit density, filling factor, and power conversion efficiency. These results indicate the feasibility of the rubbing method can be applicable to high-efficiency OPV cells.

Synthesis and Photovoltaic Properties of A Dithieno[6,5-b：10,11-b＇]-8H-Cyclopentyl[1,2-b：4,3-b＇]Diphenanthrene based Donor-Acceptor Alternating Copolymer

A novel fused nonacyclic monomer of dithieno[6,5-b：10,11-b＇]-8 H-cyclopentyl[1,2-b：4,3-b＇]diphenanthrene（DTCPDP） was synthesized by combining the structural features of ladder-type and multiple fused multi-cyclic aromatics. DTCPDP has a single sp3-hybridized carbon bridge between fused multi-cyclic aromatics. The copolymerization of DTCPDT with the electron accepting unit of 4,7-di（thiophen-2-yl）benzo[c][1,2,5]thiadiazole（DTBT） via Stille coupling afforded a novel donor-acceptor（D-A） alternating copolymer PDTCPDT-DTBT. The copolymer exhibited good chemical and thermal stabilities, with an optical band gap of 1.82 eV and a low-lying highest occupied molecular orbital（HOMO） energy level of-5.32 eV. When the copolymer was incorporated into polymer： fullerene（PC_（71）BM） blends to fabricate bulk heterojunction polymer solar cell devices, the devices exhibited a moderate maximum power conversion efficiency（PCE） of 5.90%.

Orthogonal Solubility in Fully Conjugated Donor-Acceptor Block Copolymers： Compatibilizers for Polymer/Fullerene Bulk-Heterojunction Solar Cells

Donor-acceptor （D-A） type fully conjugated block copolymer systems have been rarely reported due to the challenges in synthetic approaches to prepare well-defined low-polydispersity products. In this work, fully conjugated block copolymers are synthesized in a one-pot reaction through Stille coupling polycondensation, by utilizing the end-functional polymer copolymerization method. End-functional P3HT are copolymerized with AA （2,7-dihromo-9-（heptadecan-9-yl）-9H- carbazole） and BB （4,7-bis（5-（trimethylstannyl）thiophen-2-yl）benzo[c][1,2,5]thiadiazole, TBT） type monomers, respectively. The orthogonal solubility between the very soluble P3HT donor and the insoluble PCDTBT acceptor block improves the purity of block copolymers as well as distinct nano-scale phase-separation compared with other reports on miscibility of donor and acceptor polymer block. Further purification via preparative GPC is carried out to remove the excess of unreacted P3HT and free PCDTBT as well as to achieve low polydispersity of block copolymers. The chemical structure of the P3HT- b-PCDTBT block copolymers are verified via IH-NMR, and further confirmed by FTIR spectra. The block copolymer shows broad absorption and moderate optical band gap of 1.8 eV. Furthermore, the fully conjugated block copolymer films exhibit significant fine structures, much smoother film morphology compared to P3HT/PCDTBT polymer blends. By adding a small amount of block copolymer P3HT-b-PCDTBT as a compatibilizer into the bulk-heterojunction of P3HT：PC61BM blends, polymer solar ceils with an 8% increase of short circuit current （Jse） and 10% increase of power conversion efficiency （PCE） are achieved owing to the improvement of the active-layer film morphology. To the best of our knowledge, this is the first report on donor-acceptor type fully conjugated block copolymer as an effective ternary additive in polymer： fullerene bulk heterojunction solar cells.

Semiconducting Carbon Nanotube/Fullerene Blended Heterojunctions for Photovoltaic Near-Infrared Photon Harvesting

We demonstrate that the near-infrared （NIR） absorptivity of semiconducfing single-walled carbon nanotubes （s-SWCNTs） can be harnessed in blended heterojunctions with the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester （PCBM）. Photogenerated charge separation is efficiently driven by the ultrahigh interracial area of the blends and the favorable energy offsets between the two materials. NIR-sensitive photovoltaic and photodetector devices utilizing the stack （indium tin oxide/ca. 10 nm s-SWCNT：PCBM/100 nm CJ10 nm 2,9- dimethyl-4,7-diphenyl-],10-phenanthroline （BCP）/Ag） were fabricated with NIR power conversion efficiencies 〉1.3% and peak, zero bias external quantum efficiency of 18% at λ = 1205 nm.

The improved performance in the ternary bulk heterojunction solar cells

The ternary blend films have been fabricated via adding 4,4'-N,N'-dicarbazole-biphenyl(CBP,a hole transport material widely used in organic light emitting diodes) into the poly(3-hexylthiophene):[6,6]-phenyl C 61-butyric acid methyl ester(P3HT:PCBM).Despite the wide bandgap(3.1 eV) of the CBP,the solar cell utilizing the optimized P3HT:PCBM:CBP blend film showed an increase of 16% in power conversion efficiency and 25% in short-circuit current than the compared standard P3HT:PCBM blend film.This is attributed to the fact that the addition of the CBP could enhance the aggregation of the P3HT chains and thereby reduce the hole-electron recombination at the interface of P3HT and PCBM.We provide a simple,effective way to improve the performance of P3HT based bulk heterojunction solar cells.