This PDF file contains the front matter associated with SPIE Proceedings Volume 8830, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

We demonstrated enhanced efficiency in small molecule organic photovoltaic devices using dual organic interfacial layers of PEDOT:PSS followed by tetracene between the ITO anode and the organic donor material. The use of a small molecular templating layer, such as tetracene, proved to increase the molecular stacking of the subsequent phthalocyanine (Pc) based donor materials. Upon application in planar heterojunction devices of ZnPc and C₆₀, an enhancement of over 80 percent in the donor contribution to the external quantum efficiency was observed attributed to the combination of exciton blocking by the higher band gap tetracene layer and enhanced exciton diffusion and charge transport resulting from the increased crystallinity.

Stability of organic photovoltaic devices is a limiting factor for their commercialization and still remains a major challenge whilst power conversion efficiencies are now reaching the minimum requirements. The inverted organic solar cell architecture shows promising potential for improving significantly the cells working lifetime however, often when solution processed ZnO is used as electron extraction layer (EEL), a light soaking step is required before the device reaches a non-permanent maximum performance. Here we show that by doping ZnO with Sr or Ba using sol-gel processing the light-soaking step is circumvented. In a model poly [3-hexylthiophene] (P3HT): [6, 6]-Phenyl C60 butyl acid methyl ester (PCBM) system we obtain EQE 55% before UV exposure for ZnSrO or ZnBaO EELs as compared to 10% for undoped ZnO EEL. We have investigated the origin of this improvement by comparing the response to UV light of doped and undoped ZnO. Characterization includes electrical conductivity and x-ray photoemission spectroscopy studies on thin films, current-voltage experiments and electroabsorption (EA) spectroscopy to probe the built-in field in the devices. We will discuss how the results obtained and in particular the higher effective built-in field in doped ZnO devices (1.5V) compared to a ZnO device (0.5V) can help interpret the mechanism behind the device performance improvement with Sr and Ba doping of ZnO.

Bulk heterojunction (BHJ) polymer solar cells (PSCs) that can be fabricated by solution processing techniques are under intense investigation in both academic institutions and industrial companies because of their potential to enable mass production of flexible and cost-effective alternative to silicon-based solar cells. A combination of novel polymer development, nanoscale morphology control and processing optimization has led to over 8% of power conversion efficiencies (PCEs) for BHJ PSCs with a conventional device structure. Attempts to develop PSCs with an inverted device structure as required for achieving high PECs and good stability have, however, met with limited success. Here, we report that (1) solution-processed zinc oxide (ZnO) thin film as an electron extraction layer for inverted polymer solar cells. Operated at room temperature, no obviously degradation was observed from the PSCs with ZnO layer after continuously illuminating the devices for 4 hours. However, a significantly degradation was observed from the PSCs without ZnO buffer layer after illuminating the devices only for 1 hour. Furthermore, PSCs with ZnO buffer layer also show very good shelf stability; only 10 % degradation observed in PCEs after 6 months; (2) a high PCE of 8.4% under AM1.5G irradiation was achieved for BHJ PSCs with an inverted device structure. This high efficiency was obtained through interfacial engineering of solution-processed electron extraction layer, ZnO, leading to facilitate electron transport and suppress bimolecular recombination. All these results provided an important progress for solution-processed PSCs, and demonstrated that PSCs with an inverted device structure are comparable with PSCs with the conventional device structure.

The authors present experimental results on mechanically stacked organic solar modules and their advantage over standard tandem architectures. A four-terminal configuration of two single junction modules with complementary absorbing active layers uses the more efficient energy conversion of a tandem structure without the necessity of matching currents or voltages of electrically connected subcells. The presented combination of semitransparent and opaque solar cells consists of solution processed polymer-fullerene blends as active materials. A cost-effective mechanical scribing process is applied for the patterning of the deposited layers. The best devices have an efficiency of over 6.5% on an aperture area of 16 cm² which equals a gain of 30% over the best single junction module fabricated by the same process. Optical simulations demonstrate a 32% increased annual energy output of a mechanically stacked device in comparison to a monolithic tandem structure using an equivalent geometry.

Schottky junction organic solar cells were recently introduced and have demonstrated surprising performance with high open circuit voltage and short circuit current. In this study, the formation of the band bending region and charge extraction from the donor molecules in C₆₀ and C₇₀ based Schottky junction solar cells are investigated. Band bending was observed when the ITO anode was treated with CF₄O₂ and O₂ plasma. These cells demonstrated similar performance to cells with MoO₃ coated ITO, which are more common in literature. The presence of donor material at the anode interface was also found to decrease the open circuit voltage of the cell by decreasing the number of fullerene molecules at the anode and consequently, decreasing the extent of band bending. The short-circuit current of the Schottky junction solar cells increased when a thin layer of neat fullerene was introduced between the ITO anode and the donor doped mixed layer by reducing exciton quenching at the anode interface. Finally, hole extraction from the donor was found to be efficient up to 5 nm away from the anode interface. If donor molecules were placed beyond this distance the fill factor dropped precipitously along with the overall cell efficiency.

Photocrosslinking is known as a suitable method for patterning organic semiconductors in organic light emitting diodes. We extend this concept to the field of organic solar cells using conjugated polymers bearing sidechains with photocrosslinkable oxetane units. By UV irradiation in the presence of a photo acid generator the oxetane groups polymerize, leading to the formation of a densely crosslinked, and thus insoluble, network of a low-bandgap polymer. In this paper we present the synthesis of two novel photocrosslinkable low-bandgap polymers PFDTBTOx and PCDTBTOx and discuss several strategies for the fabrication of organic solar cells taking advantage of the novel crosslinkable materials.

Expanding the spectral absorption breadth and efficiently harvesting excitons are crucial towards creating highly efficient polymer solar cells. Here we describe a strategy to realize broad-band light harvesting in poly(3-hexylthiophene) (P3HT)-based solar cells. We introduce the use of squaraine dye molecules that play a dual role towards improving P3HT-based solar cells. The first benefit is an increase in the spectral absorption in the near infrared region. The second advantage is the collection of excitons close to the interfacial heterojunctions via Förster resonance energy transfer (FRET). Unlike traditional multi-blend systems, where each donor works independently in separate spectral responses, FRET-based systems enable the effective use of multiple donors with significant improvements in light absorption and conversion. Ultrafast transient absorption experiments show that the excitation energy from P3HT can be transferred rapidly (within a few picoseconds) and efficiently (up to 96%) to the squaraine via FRET. As a result, the overall power conversion efficiency is improved. This architecture opens up a new paradigm towards transformative improvements of polymer solar cells.

An empirically based, open source, optoelectronic model is constructed to accurately simulate organic photovoltaic (OPV) devices. Bulk heterojunction OPV devices based on a new low band gap dithienothiophene- diketopyrrolopyrrole donor polymer (P(TBT-DPP)) are blended with PC₇₀BM and processed under various conditions, with efficiencies up to 4.7%. The mobilities of electrons and holes, bimolecular recombination coefficients, exciton quenching efficiencies in donor and acceptor domains and optical constants of these devices are measured and input into the simulator to yield photocurrent with less than 7% error. The results from this model not only show carrier activity in the active layer but also elucidate new routes of device optimization by varying donor-acceptor composition as a function of position. Sets of high and low performance devices are investigated and compared side-by-side.

Ternary blend solar cells are considered presents one route to construct the devices with a broad absorption of the solar spectrum. However, the morphological studies on such mixtures have been limited. Here, the morphology of P3HT/PCPDTBT/PC₆₁BM ternary blend thin films were studied. By adjusting the blending ratio of two polymers, P3HT molecular weight, thermal annealing time and spin-coating solvents, the crystallinity of both polymers and phase separation among each component were controlled. It was found that a high crystallinity of the polymers is important for good device performance. Due to the crystallization of the polymers and the immiscibility between P3HT and PCPDTBT, a hierarchical morphology was generated that mimicked a tandem cell connected in parallel and extended the absorption. In addition, the amorphous PCPDTBT was found to guide the orientation of P3HT nanocrystals before they merged into fibrils, and it also served as a photosensitizer to form a cascade energy level alignment. Therefore, the two polymers worked synergistically to achieve the improved device performance relative to the binary reference.

Organic solar cells are a favorable alternative to their inorganic counterparts because the functional layers of these devices can be processed with printing or coating on a large scale. In this study, a novel absorber polymer was synthesized, blended with fullerene and deposited with inkjet printing for solar cell applications. A fluorene based terpolymer with dialkyl substituted diphenyl-dithienylbenzopyrazine and triphenylamine units was synthesized by Suzuki polymerization with high molecular weights. The introduction of dialkyl substituted diphenyldithienylbenzopyrazine in the fluorene main chain leads to LUMO-energy level of -3.1 eV and to an open circuit voltage of 0.96 V in solar cells. All the requirements were fulfilled to achieve absorber polymers with high efficiencies including a HOMO energy level which is lower than -5.2 eV, a band gap in the range of 1.3-1.9 eV and a hole mobility in OTFTs greater than 1 x 10-3cm2/Vs. Solar cells with printed layers were compared to those with spin coated films in order to evaluate inkjet printing as a thin film deposition method. Efficiency values of 3.7% were found for devices with inkjet printed layers or spin coated layers when using chlorinated solvents. In order to be able to use inkjet printing on a large scale, hazardous, chlorinated solvents should be avoided when depositing the functional materials. Anisol/tetralin was used as an alternative solvent system. It was found that devices prepared from the chlorine-free system showed only slightly lower efficiencies of 2.7% with respect to the chlorinated system. A coarser phase separation was found with energy filtered transmission electron microscopy plasmon mapping which most likely resulted in the performance differences for the chlorinated and chlorine-free solvent systems. This paper, originally published on 17 October 2013, was replaced with the correct version on 30 January 2014. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.

In this Proceedings paper, we report on the synthesis of a family of polythiophene-based conjugated polyelectrolytes, both homopolymers and random copolymers varying in the building block ratio and counter ions, toward a better fundamental understanding of the structure-property relations of these ionic derivatives in organic photovoltaics. One of the ionic homopolymers was successfully implemented as a donor material in fully solution-processed efficient bi-layer solar cells (up to 1.6% PCE in combination with PC₇₁BM) prepared by the low impact meniscus coating technique. On the other hand, these imidazolium-substituted polythiophenes were also applied as materials for electron transport layers (ETLs), boosting the I-V properties of PCDTBT:PC₇₁BM solar cell devices up to average PCE values of 6.2% (~20% increase), which is notably higher than for previously reported ETL materials. Advanced scanning probe microscopy techniques were used to elucidate the efficiency enhancing mechanism.

We propose an optimization algorithm to design multilayer antireflection (AR) structure, which has robustness against variations in layer thicknesses, for organic photovoltaic cells. When a set of available materials are given, the proposed method searches for the material and thickness of each AR layer to maximize the short-circuit current density (Jsc). This algorithm allows for obtaining a set of solutions, including optimal and quasi-optimal solutions, at the same time, so that we can clearly make comparison between them. In addition, the effects of deviations in the thicknesses of the AR layers are examined for the (quasi-)optimal solutions obtained. The expectation of the decrease in the AR performance is estimated by calculating the changes in Jsc when the thicknesses of all AR layers are varied independently. We show that some of quasi-optimal solutions may have simpler layer configuration and can be more robust against the deviations in film thicknesses, than the optimal solution. This method indicates the importance of actively searching valuable, nonoptimal solutions for practical design of AR films. We also discuss the optical conditions that lead to light absorption in the back metal contact and the effects of changing active layer thicknesses.

We analyze the photocurrent spectra for bulk heterojunction organic solar cells having a range of active layer thicknesses. Normalized by the number of incident photons, the photocurrent peak red shifts with respect to the absorption maximum as the sample thickness increases. The shift in photocurrent peak can be understood by comparing the carrier extraction for bulk versus surface generated carriers. Fitting to the spectra provides a measure of the electron collection length and the surface enhanced recombination. The variation in the electron collection length with contacting parameters is used as a guide for optimization of the device structure. Furthermore, a photocurrent analysis has been done for different active layer thicknesses with different interfacial layers. Again the model fit for this comparison provides an estimate for carrier recombination distance that fits well with experimental data. For this study a common conjugated polymer regioregular poly(3-hexylthiophene) (P3HT) is used while poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT-PSS), P3HT and Al are used as interfacial layers.

Polymer solar cells (PSC’s) have received much attention as a promising clean and green energy technology and the power conversion efficiency have steadily increased. There are several ways to improve the device efficiency of PSC, such as changing the active layer, insertion of the electron transport layer and the anode buffer layer. Among the several anode buffer layer materials, Poly(3,4-Ethylenedioxythiophene):Poly(styrene sulfonate) (PEDOT:PSS) is widely used as anode buffer layer due to its high transparency in the visible region, high thermal stability and mechanical flexibility. However, PEDOT:PSS suffers a problem of low conductivity and limits the device application. In this report, we present the preparation of PEDOT:PSS hole transport layer through a secondary doping with flouro compounds such as hexafluoroacetone (HFA) and hexafluoroisoproponal (HIPA) with various concentrations by spin coating technique. High performance of the hole transport layer is attributed to preferential phase segregation of PEDOT:PSS with HFA and HIPA solvent mixture treatment method. The improved performance of PSC was dependent on the structure of organic solvents and the concentration of flouro compounds in PEDOT:PSS solution. Using these optimized buffer layer, conjugated polymer solar cells with a Poly9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3- benzothiadiazole-4,7-diyl-2,5 thiophenediyl] polymer:[6,6]-phenyl-C71-butyric acid methyl esters (PCDTBT:PC71BM) bulk heterojunction have been produced. A detailed analysis of the surface morphology and optical studies are presented. The obtained results show that PEDOT:PSS optimized with HFA and HIPA organic solvents can be a very promising candidate for transparent anode buffer layer in the low cost organic solar cell devices.

Bulk heterojunction (BHJ) solar cells have made great progress during the past decade and consequently are now attracting extensive academic and commercial interest because of their potential advantages: lightweight, flexible, low cost, and high-throughput production. We report on the fabrication of poly((4,8-diethylhexyloxyl) benzo([1,2-b:4,5- b′]dithiophene)-2,6-diyl)-alt-((5-octylthieno[3,4-c]pyrrole-4,6-dione)-1,3-diyl) /[6,6]-phenyl-C71-butyric acid methyl ester blend active layer using airbrush spray coating method in different solvents. The parameters such as spraying time, substrate-nozzle distance for the deposition of active layers were analysed. Optical absorption of the active layers was analyzed using UV-visible spectral studies in the wavelength range from 300 to 800 nm. The surface morphology of the active layers deposited with different parameters was examined using Atomic Force Microscopy. The current densityvoltage (J-V) characteristics of photovoltaic cells were measured under the illumination of simulated solar light with 100 mW/cm² (AM 1.5G) by an Oriel 1000 W solar simulator. We also notice that both the bottom-up and top-down approaches have played important roles in advancing our fundamental understanding of this new class of nanostructures. Finally we attempt to look into the future and offer our personal opinions on what the future trends will be in organic solar cell research.

We performed numerical simulations of transient photocurrents in organic thin films, in conjunction with experiments. This enabled us to quantify the contribution of multiple charge generation pathways to charge carrier photogeneration, as well as extract parameters that characterize charge transport, in functionalized anthradithiophene (ADT-TES-F) films prepared using two different deposition methods: drop casting on an untreated substrate and spin casting on a pentauorobenzenethiol (PFBT)-treated substrate. These deposition methods yielded polycrystalline lms with considerably larger grain sizes in the case of the spin cast lm. In both drop cast and spin cast films, simulations revealed two competing charge photogeneration pathways: fast charge generation on a picosecond (ps) or sub-ps time scale with efficiencies below 10%, and slow charge generation, on the time scale of tens of nanoseconds, with efficiencies of 11-12% in drop cast and 50-60% in spin cast films, depending on the applied electric field. The total charge photogeneration efficiency in the spin cast sample was 59-67% compared to 14-20% in the drop cast sample, whereas the remaining 33-41% and 80-86%, respectively, of the absorbed photon density did not contribute to charge carrier generation on these time scales. The spin cast film also exhibited higher hole mobilities, lower trap densities, shallower traps, and lower charge carrier recombination, as compared to the drop cast lm. As a result, the spin cast lm exhibited higher photocurrents despite a considerably lower lm thickness (and thus reduced optical absorption and cross section of the current flow).

In this study, a novel bifacially active transparent dye-sensitized solar cell (DSSCs) assembled with a transparent poly(3,4-ethylenedioxythiophene) (PEDOT) counter electrode and a colorless iodine-free polymer gel (IFPG) electrolyte was developed. The IFPG electrolyte was prepared by employing an ionic liquid (1,2-dimethyl-3-propylinmidazolium iodide, DMPII) as the charge transfer intermediate and a polymer composite as the gelator without the addition of iodine, exhibiting high conductivity and non-absorption characters. PEDOT electrodes were prepared via a facile electro-polymerization method. By controlling the amount of polymerization charge capacity, we optimized the PEDOT electrodes with high transparency and a favorable activity for catalyzing the IFPG electrolyte. The bifacial DSSCs device fabricated by this kind of transparent PEDOT electrode and colorless IFPG electrolyte showed a power conversion efficiency (PCE) of 6.35% and 4.98% at 100 mW cm^-2 AM1.5 illumination corresponding to front- and rear-side illumination. It is notable that the PCE under rear-side illumination approaches 80% that of front-side illumination. Moreover, the device shows excellent stability as confirmed by aging test. These promising results highlight the enormous potential of this transparent PEDOT CE and colorless IFPG electrolyte in scaling up and commercialization of low cost and effective bifacial DSSCs.

Vacuum-deposited small molecule organic solar cells (OSCs) with various electron extraction layers (EELs) and hole extraction layers (HELs) are studied. Upright OSCs are examined for their efficiencies, and for their photo- and thermal-stabilities. It is shown that the EEL conducts only because of the subsequent top electrode metal deposition, and that exciton blocking characteristics of the EEL alone are not sufficient to grant good solar cell characteristics. To this end, wide bandgap organic HELs cannot be placed between the bottom electrode and active organic layers to achieve the same effect as wide bandgap organic EELs at the opposite end of the device. Inverted vacuumdeposited OSCs, which place the HEL adjacent to the top metal anode, can make use of a wide bandgap organic HEL in combination with a thin MoO₃ layer to achieve good device performance. Thick MoO₃ HELs alone are also shown to be suitable for inverted vacuum-deposited OSCs, thus simplifying device fabrication. For these inverted devices, the thickness of the HEL required to achieve good device performance is found to be substantially higher than the equivalent EEL thickness in a standard upright device.

The high cost of electricity produced by solar cells compared with electricity from other energy sources inhibits a more widespread adoption of solar energy. Here, a low-cost monolithic all-solid-state dye-sensitized solar cell (DSSC) was developed with a mesoscopic carbon counter electrode (CE). Based on the design of a triple layer structure, the TiO2 working electrode layer, ZrO2 spacer layer and carbon counter electrode (CE) layer are constructed on a single conducting glass substrate by screen-printing. With a vacuum pore-filling technique, solid-state materials such as PEO/PVDF polymer composite, poly(3-hexylthiophene) (P3HT) and 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)- 9,9’-spirobifluorene (spiro-OMeTAD) hole transport material (HTM) could effectively infiltrate the multilayer thick films to assemble all-solid-state devices. The high surface area and large pore volume favor the penetration of the solidstate electrolyte materials and could reduce the resistance of the interface between CE and solid-state electrolyte. Correspondingly, efficiency up to 3.23% was obtained with polymer composite electrolyte and the dye of N719. With the dye of D102, efficiencies of 3.11% and 3.45% were obtained for the HTMs of P3HT and spiro-OMeTAD based electrolytes. In addition, a mesoscopic methylammonium lead iodide (CH₃NH₃PbI₃) perovskite/TiO2 heterojunction solar cell was developed based on the monolithic structure and showed an efficiency of up to 6.53%. This design for monolithic DSSC with a carbon CE presents a promising commercial application prospect for this photovoltaic technology.

Solvents are often used in the active layer formation process in many photonic applications such as organic solar cell, organic led, organic wavelength conversion and photorefractive materials. In this contribution, multiscale modeling and simulation was used to reveal phase separation in P3HT-PCBM at mesoscale level due to attractionrepulsion between different organic functional groups of active ingredient molecules and solvent molecules. Force field parameters for mesoscale calculation were obtained from dynamic mapping of results from molecular dynamic simulations. DFT calculation was used to describe energy changes of active ingredient molecules due to surrounded residual solvent molecules. The simulation results from no solvent, chloroform, dichlorobenzene and chlorobenzene cases indicated that chlorobenzene exhibits strong attraction with fullerene of PCBM and strong repulsion with all the other functional groups, therefore leads to least phase segregation. DFT calculation showed that residual solvent molecules can slightly lower the energy but do not alter the value of band gap.

We report on development of flexible PCPDTBT:PCBM solar cells with integrated diffraction gratings on the bottom electrodes. The presented results address PCPDTBT:PCBM solar cells in an inverted geometry, which contains implemented grating structures whose pitch is tuned to match the absorption spectra of the active layer. This optimized solar cell structure leads to an enhanced absorption in the active layer and thus improved short-circuit currents and power conversion efficiencies in the fabricated devices. Fabrication of the solar cells on thin polyimide substrates which are compatible with the lithographically processed grating structures are done in order to obtain the efficiency enhancement in thin, flexible devices.

Copper(II), Cobalt (II) and Iron (II) complexes as photosensitizer on Dye Sensitized Solar Cell (DSSC) had been investigated. The aim of this research is to find out the respond addition of those dyes on FTO/TiO2 (FTO = fluorine Tin Oxide) thin film to visible light and the effect of various third row complexes to DSSC performance. Slip casting method was used to fabricate FTO/TiO2 and FTO/carbon thin film. The result from FTO/TiO2 UV-Vis spectra show no absorption on visible light. Dye solution was synthesized from free metal ions of Cu(II), Co(II), and Fe(II) in methanol with diphenylamine (dpa), 2,2,bypiridine (bpy), 1,10, phenathroline (phen), 4,4’-dicarboxylic acid-2,2’-bipyridine (dcbq), and anthocyanin (ant) ligands, respectively. UV-Vis spectrophotometry was used to identify FTO/TiO2/dye with various sensitizer dyes. The performance of DSSC was determined by I (current) - V (voltage) curve using Keithley 2602 A System Source. In this research, DSSCs are able to convert photon energy become electrical energy. Dye used in DSSC is greatly effect in photon to current efficiency (IPCE). The greater absorption in visible region of alternative dye used gains higher IPCE spectra. TiO2 character can help spread the absorption in whole visible region. The nanosize mesoporous TiO2 of TiO2/SiPA/CoII-PAR (SiPA = silylpropilamine) have greater value than P25 TiO2/SiPA-Co^II-PAR. The SiPA/Fe^II-PAR and SiPA/Co^II-PAR dyes are better dye than tpa.

Recently, excellent solar cell device performances have been achieved with solution-processed small-molecule donor materials. Small molecules have well defined structures and thus allow better control of self-assembly in the solid state. However, the easy formation of H-type aggregates and lack of strong interactions between nanodomains could limit charge transport, device performance, and long-term stability. We have recently explored the synthesis of ring-protected small molecules (with rings surrounding the center of the molecules), studied the intermolecular interactions in solution and solid state, and conducted preliminary solar cell device fabrications. It has been found that the molecules behave very differently from conventional flat small molecules in both solution and solid states. Proton NMR study of solutions of different concentrations revealed the presence of strong intermolecular interactions as a result of absence or shortage of open-ended alkyl side chains; however, such strong interactions do not lead to precipitation of the molecules even at high concentrations. Excellent films are routinely obtained from the neat small molecules despite the much reduced number of solubilizing groups. The New findings strongly suggest that ring protection is an effective strategy to avoid Haggregation and maintain strong pi-pi interactions simultaneously. Such materials are expected to form head-tail selfassemblies that will open new possibilities for small molecule organic materials. Conceptually, thin films of such materials are potentially more isotropic in charge transport than conventional small molecule and polymer films, a property desirable for photovoltaics and some other optoelectronic applications.

Several important developments in the field of Organic Spintronics and magnetic field effect in organic optoelectronic devices will be surveyed and discussed. Organic Spintronics: We demonstrated spin organic light emitting diodes using two FM injecting electrodes, where the electroluminescence depends on the mutual orientation of the electrode magnetization directions. This development has opened up research studies into organic spin-valves in the space-charge limited current regime. Spin effects in organic photovoltaic solar cells: We demonstrated that spin 1/2 radical additives to donor-acceptor (D-A) blends improve the power conversion efficiency via resonant spin-spin interaction between the radicals and charge-transfer excitons at the D-A interfaces.