Compositional Dependence of the Performance of Poly(p‐phenylene vinylene):Methanofullerene Bulk‐Heterojunction Solar Cells

The dependence of the performance of OC₁C₁₀‐PPV:PCBM (poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene):methanofullerene [6,6]‐phenyl C₆₁‐butyric acid methyl ester)‐based bulk heterojunction solar cells on their composition has been investigated. With regard to charge transport, we demonstrate that the electron mobility gradually increases on increasing the PCBM weight ratio, up to 80 wt.‐%, and subsequently saturates to its bulk value. Surprisingly, the hole mobility in the PPV phase shows an identical behavior and saturates beyond 67 wt.‐% PCBM, a value which is more than two orders of magnitude higher than that of the pure polymer. The experimental electron and hole mobilities were used to study the photocurrent generation of OC₁C₁₀‐PPV:PCBM bulk‐heterojunction (BHJ) solar cells. From numerical calculations, it is shown that for PCBM concentrations exceeding 80 wt.‐% reduced light absorption is responsible for the loss of device performance. From 80 to 67 wt.‐%, the decrease in power conversion efficiency is mainly due to a decreased separation efficiency of bound electron–hole (e–h) pairs. Below 67 wt.‐%, the performance loss is governed by a combination of a reduced generation rate of e–h pairs and a strong decrease in hole transport.     

Hole Transport in Poly(phenylene vinylene)/Methanofullerene Bulk‐Heterojunction Solar Cells

A fundamental limitation of the photocurrent of solar cells based on a blend of poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene) (MDMO‐PPV) and [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) is caused by the mobility of the slowest charge‐carrier species, the holes in the MDMO‐PPV. In order to allow the experimentally observed photocurrents electrostatically, a hole mobility of at least 10^–8 m² V^–1 s^–1 is required, which exceeds the observed hole mobility in pristine MDMO‐PPV by more than two orders of magnitude. However, from space‐charge‐limited conduction, admittance spectroscopy, and transient electroluminescence measurements, we found a hole mobility of 2 × 10^–8 m² V^–1 s^–1 for the MDMO‐PPV phase in the blend at room temperature. Consequently, the charge‐carrier transport in a MDMO‐PPV:PCBM‐based solar cell is much more balanced than previously assumed, which is a necessary requirement for the reported high fill factors of above 50 %.     

Nanoscale Morphology of Conjugated Polymer/Fullerene‐Based Bulk‐ Heterojunction Solar Cells

The relation between the nanoscale morphology and associated device properties in conjugated polymer/fullerene bulk‐heterojunction “plastic solar cells” is investigated. We perform complementary measurements on solid‐state blends of poly[2‐methoxy‐5‐(3,7‐dimethyloctyloxy)]‐1,4‐phenylenevinylene (MDMO‐PPV) and the soluble fullerene C₆₀ derivative 1‐(3‐methoxycarbonyl) propyl‐1‐phenyl [6,6]C₆₁ (PCBM), spin‐cast from either toluene or chlorobenzene solutions. The characterization of the nanomorphology is carried out via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while solar‐cell devices were characterized by means of current–voltage (I–V) and spectral photocurrent measurements. In addition, the morphology is manipulated via annealing, to increase the extent of phase separation in the thin‐film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials under thermal treatment. Furthermore the photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase‐separated blends cast from toluene. For toluene‐cast films the scale of phase separation depends strongly on the ratio of MDMO‐PPV to PCBM, as well as on the total concentration of the casting solution. Finally we observe small beads of 20–30 nm diameter, attributed to MDMO‐PPV, in blend films cast from both toluene and chlorobenzene.     

Origin of the Reduced Fill Factor and Photocurrent in MDMO‐PPV:PCNEPV All‐Polymer Solar Cells

The photogeneration mechanism in blends of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV) and poly[oxa‐1,4‐phenylene‐(1‐cyano‐1,2‐vinylene)‐(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene)‐1,2‐(2‐cyanovinylene)‐1,4‐phenylene] (PCNEPV) is investigated. The photocurrent in the MDMO‐PPV:PCNEPV blends is strongly dependent on the applied voltage as a result of a low dissociation efficiency of the bound electron–hole pairs. The dissociation efficiency is limited by low carrier mobilities, low dielectric constant, and the strong intermixing of the polymers, leading to a low fill factor and a reduced photocurrent at operating conditions. Additionally, electrons trapped in the PCNEPV phase recombine with the mobile holes in the MDMO‐PPV phase at the interface between the two polymers, thereby affecting the open‐circuit voltage and increasing the recombination losses. At an intensity of one sun, Langevin recombination of mobile carriers dominates over trap‐assisted recombination.     

Ultrafast Hole‐Transfer Dynamics in Polymer/PCBM Bulk Heterojunctions

Ultrafast dynamics of the hole‐transfer process from methanofullerene to a polymer in a polymer/PCBM bulk heterojunction are directly resolved. Injection of holes into MDMO‐PPV is markedly delayed with respect to [60]PCBM excitation. The fastest component of the delayed response is attributed to the PCBM–polymer hole‐transfer process (30 ± 10 fs), while the slower component (～150 fs) is provisionally assigned to energy transfer and/or relaxation inside PCBM nanoclusters. The charge generation through the hole transfer is therefore as fast and efficient as through the electron‐transfer process. Exciton harvesting efficiency after PCBM excitation crucially depends on the concentration of the methanofullerene in the blend, which is related to changes in the blend morphology. Ultrafast charge generation is most efficient when the characteristic scale of phase separation in the blend does not exceed ～20 nm. At larger‐scale phase separation, the exciton harvesting dramatically declines. The obtained results on the time scales of the ultrafast charge generation after PCBM excitation and their dependence on blend composition and morphology are instrumental for the future design of fullerene‐derivative‐based photovoltaic devices.     

Hybrid Solar Cells Using a Zinc Oxide Precursor and a Conjugated Polymer

We describe a new method towards bulk‐heterojunction hybrid polymer solar cells based on composite films of zinc oxide (ZnO) and a conjugated polymer poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV). Spin‐coating diethylzinc as a ZnO precursor and MDMO‐PPV from a common solvent at 40 % humidity and annealing at 110 °C provides films in which crystalline ZnO is found to be intimately mixed with MDMO‐PPV. Photoluminescence and photoinduced spectroscopy demonstrate that photoexcitation of these hybrid composite films results in a fast and long‐lived charge transfer from the polymer as a donor to ZnO as ato be obtained n acceptor. Using the ZnO‐precursor method, hybrid polymer solar cells have been made with an estimated air‐mass of 1.5 (AM 1.5) energy conversion efficiency of 1.1 %. This new method represents a fivefold improved performance compared to similar hybrid polymer solar cells based on amorphous TiO₂.     

Effects of Photo‐oxidation on the Performance of Poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene]:[6,6]‐Phenyl C61‐Butyric Acid Methyl Ester Solar Cells

A study of the photo‐oxidation of films of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV) blended with [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM), and solar cells based thereon, is presented. Solar‐cell performance is degraded primarily through loss in short‐circuit current density, J_SC. The effect of the same photodegradation treatment on the optical‐absorption, charge‐recombination, and charge‐transport properties of the active layer is studied. It is concluded that the loss in J_SC is primarily due to a reduction in charge‐carrier mobility, owing to the creation of more deep traps in the polymer during photo‐oxidation. Recombination is slowed down by the degradation and cannot therefore explain the loss in photocurrent. Optical absorption is reduced by photo‐bleaching, but the size of this effect alone is insufficient to explain the loss in device photocurrent.     

Ambipolar Charge Transport in Films of Methanofullerene and Poly(phenylenevinylene)/Methanofullerene Blends

Herein, we report experimental studies of electron and hole transport in thin films of [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) and in blends of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV) with PCBM. The low‐field hole mobility in pristine MDMO‐PPV is of the order of 10^–7 cm² V^–1 s^–1, in agreement with previous studies, whereas the electron mobility in pristine PCBM was found by current‐density–voltage (J–V) measurements to be of the order of 10^–2 cm² V^–1 s^–1, which is about one order of magnitude greater than previously reported. Adding PCBM to the blend increases both electron and hole mobilities, compared to the pristine polymer, and results in less dispersive hole transport. The hole mobility in a blend containing 67 wt.‐% PCBM is at least two orders of magnitude greater than in the pristine polymer. This result is independent of measurement technique and film thickness, indicating a true bulk property of the material. We therefore propose that PCBM may assist hole transport in the blend, either by participating in hole transport or by changing the polymer‐chain packing to enhance hole mobility. Time‐of‐flight mobility measurements of PCBM dispersed in a polystyrene matrix yield electron and hole mobilities of similar magnitude and relatively non‐dispersive transport. To the best of our knowledge, this is the first report of hole transport in a methanofullerene. We discuss the conditions under which hole transport in the fullerene phase of a polymer/fullerene blend may be expected. The relevance to photovoltaic device function is also discussed.     

Influence of side chain symmetry on the performance of poly(2,5-dialkoxy-p-phenylenevinylene): fullerene blend solar cells

We report on studies of poly-(2,5-dihexyloxy-p-phenylenevinylene) (PDHeOPV), a symmetric side-chain polymer, as a potential new donor material for polymer:fullerene blend solar cells. We study the surface morphology of blend films of PDHeOPV with PCBM, the transport properties of the blend films, and the performance of photovoltaic devices made from such blend films, all as a function of PCBM content. In each case, results are compared with those obtained using the asymmetric side chain polymer, poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), in order to investigate the influence of polymer side chain symmetry on solar cell performance. AFM images show that large PCBM aggregates appear at lower PCBM content (50 wt.% PCBM) for PDHeOPV:PCBM than for MDMO-PPV:PCBM (67 wt.% PCBM) blend films. Time-of-Flight (ToF) mobility measurements show that charge mobilities depend more weakly on PCBM content in PDHeOPV:PCBM than in MDMO:PPV:PCBM, with the result that at high PCBM content the mobilities in PDHeOPV:PCBM are significantly lower than in MDMO:PPV:PCBM blend films, despite the higher mobilities in pristine PDHeOPV compared to pristine MDMO-PPV. Photovoltaic devices show significantly lower power conversion efficiency (～0.93%) for PDHeOPV:PCBM (80 wt.% PCBM) blend films than for MDMO-PPV:PCBM (2.2% at 80 wt.% PCBM) blends. This is attributed to the relatively poor transport properties of the PDHeOPV:PCBM blend, which limit the optimum thickness of the photoactive layer in PDHeOPV:PCBM blend devices. The behaviour is tentatively attributed to a higher tendency for the symmetric side-chain polymer chains to aggregate, resulting in poorer interaction with the fullerene and poorer network formation for charge transport.     

Formation of Methanofullerene Cation in Bulk Heterojunction Polymer Solar Cells Studied by Transient Absorption Spectroscopy

Photogenerated charge carriers for blend films of poly[2‐methoxy‐5‐(3,7‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) have been investigated by transient absorption spectroscopy. The blend film with a low PCBM fraction (<10 wt %) exhibits a wide absorption that ranges from 900 to 1000 nm, which is characteristic of the MDMO‐PPV hole polaron and PCBM radical anion. On the other hand, the blend film with a higher PCBM fraction (> 30 wt %) exhibits a major absorption band at ∼900 nm, which is characteristic of the PCBM radical cation. For identification of charge carriers, the absorption spectrum and molar absorption coefficient of each charged species have been evaluated separately using various combinations of electron donor and acceptor materials. Consequently, the MDMO‐PPV hole polaron has been found to have a broad absorption at ∼950 nm and the PCBM radical anion and cation show a distinct absorption at 1020 and 890 nm, respectively. On the basis of these absorption spectra, the transient spectra observed for the blend films have been simulated. The spectrum for a low PCBM fraction is well reproduced by superposition of the absorption spectra of the MDMO‐PPV hole polaron and PCBM radical anion. On the other hand, the spectrum for a high PCBM fraction is well reproduced by superposition of the absorption spectra of the MDMO‐PPV hole polaron, PCBM radical anion, and PCBM radical cation, which indicates that the PCBM radical cation is formed in the blend films with PCBM at a high concentration. Possible mechanisms for the formation of the PCBM radical cation in the blend are also discussed.     

In‐Situ Compositional and Structural Analysis of Plastic Solar Cells

Bulk‐heterojunction photovoltaic cells consisting of a photoactive layer of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV) and a C₆₀ derivative, (1‐(3‐methoxycarbonyl)propyl‐1‐phenyl‐[6,6]‐methanofullerene), (PCBM), sandwiched between an indium tin oxide (ITO) anode covered with poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and an aluminum cathode have been analyzed using transmission electron microscopy (TEM) and cryogenic Rutherford backscattering spectrometry (RBS) to assess the structural and elemental composition of these devices. TEM of cross sections of fully processed photovoltaic cells, prepared using a focused ion beam, provide a clear view of the individual layers and their interfaces. RBS shows that during preparation diffusion of indium into the PEDOT:PSS occurs while the diffusion of aluminum into the polymer layers is negligible. An iodinated C₆₀ derivative (I‐PCBM) was used to determine the concentration profile of this derivative in the vertical direction of a 100 nm active layer.     

Electron Transport in a Methanofullerene

The current–voltage characteristics of methanofullerene [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM)‐based devices are investigated as a function of temperature. The occurrence of space–charge limited current enables a direct determination of the electron mobility. At room temperature, an electron mobility of μ_e = 2 × 10^–7 m² V^–1 s^–1 has been obtained. This electron mobility is more than three orders of magnitude larger than the hole mobility of donor‐type conjugated polymer poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐p‐phenylene vinylene) (OC₁C₁₀‐PPV). As a result, the dark current in PCBM/OC₁C₁₀‐PPV based devices is completely dominated by electrons. The observed field and temperature‐dependence of the electron mobility of PCBM can be described with a Gaussian disorder model. This provides information about the energetic disorder and average transport‐site separation in PCBM.     

A Low‐Bandgap Semiconducting Polymer for Photovoltaic Devices and Infrared Emitting Diodes

A novel low‐bandgap conjugated polymer (PTPTB, E_g = ∼ 1.6 eV), consisting of alternating electron‐rich N‐dodecyl‐2,5‐bis(2′‐thienyl)pyrrole (TPT) and electron‐deficient 2,1,3‐benzothiadiazole (B) units, is introduced for thin‐film optoelectronic devices working in the near infrared (NIR). Bulk heterojunction photovoltaic cells from solid‐state composite films of PTPTB with the soluble fullerene derivative [6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) as an active layer shows promising power conversion efficiencies up to 1 % under AM1.5 illumination. Furthermore, electroluminescent devices (light‐emitting diodes) from thin films of pristine PTPTB show near infrared emission peaking at 800 nm with a turn on voltage below 4 V. The electroluminescence can be significantly enhanced by sensitization of this material with a wide bandgap material such as the poly(p‐phenylene vinylene) derivative MDMO‐PPV.     

Scanning Kelvin Probe Microscopy on Bulk Heterojunction Polymer Blends

Here, correlated AFM and scanning Kelvin probe microscopy measurements with sub‐100 nm resolution on the phase‐separated active layer of polymer‐fullerene (MDMO‐PPV:PCBM) bulk heterojunction solar cells in the dark and under illumination are described. Using numerical modeling a fully quantitative explanation for the contrast and shifts of the surface potential in dark and light is provided. Under illumination an excess of photogenerated electrons is present in both the donor and acceptor phases. From the time evolution of the surface potential after switching off the light the contributions of free and trapped electrons can be identified. Based on these measurements the relative 3D energy level shifts of the sample are calculated. Moreover, by comparing devices with fine and coarse phase separation, it is found that the inferior performance of the latter devices is, at least partially, due to poor electron transport.     

Effect of Temperature and Illumination on the Electrical Characteristics of Polymer–Fullerene Bulk‐Heterojunction Solar Cells

The current–voltage characteristics of ITO/PEDOT:PSS/OC₁C₁₀‐PPV:PCBM/Al solar cells were measured in the temperature range 125–320 K under variable illumination, between 0.03 and 100 mW cm^–2 (white light), with the aim of determining the efficiency‐limiting mechanism(s) in these devices, and the temperature and/or illumination range(s) in which these devices demonstrate optimal performance. (ITO: indium tin oxide; PEDOT:PSS: poly(styrene sulfonate)‐doped poly(ethylene dioxythiophene); OC₁C₁₀‐PPV: poly[2‐methoxy‐5‐(3,7‐dimethyl octyloxy)‐1,4‐phenylene vinylene]; PCBM: phenyl‐C₆₁ butyric acid methyl ester.) The short‐circuit current density and the fill factor grow monotonically with temperature until 320 K. This is indicative of a thermally activated transport of photogenerated charge carriers, influenced by recombination with shallow traps. A gradual increase of the open‐circuit voltage to 0.91 V was observed upon cooling the devices down to 125 K. This fits the picture in which the open‐circuit voltage is not limited by the work‐function difference of electrode materials used. The overall effect of temperature on solar‐cell parameters results in a positive temperature coefficient of the power conversion efficiency, which is 1.9 % at T = 320 K and 100 mW cm^–2 (2.5 % at 0.7 mW cm^–2). The almost‐linear variation of the short‐circuit current density with light intensity confirms that the internal recombination losses are predominantly of monomolecular type under short‐circuit conditions. We present evidence that the efficiency of this type of solar cell is limited by a light‐dependent shunt resistance. Furthermore, the electronic transport properties of the absorber materials, e.g., low effective charge‐carrier mobility with a strong temperature dependence, limit the photogenerated current due to a high series resistance, therefore the active layer thickness must be kept low, which results in low absorption for this particular composite absorber.     

Anodic and Cathodic Electrochemically Generated Chemiluminescence in Conjugated Polymers

In the present contribution, we show that long‐lived electrochemically generated chemiluminescence (ECL) from conjugated polymers can be achieved in both anodic and cathodic polarizations of poly[2‐(2′‐ethylethoxy)‐5‐methoxy‐1,4‐phenylene vinylene] (MEH–PPV), when the appropriate electrochemical conditions are adopted and an adequate degree of purity for the active polymer, solvent, and supporting electrolyte is achieved. The quantum efficiency is generally higher during the anodic polarization of MEH–PPV, whereas the ECL vs. time profile depends on the nature of the supporting electrolyte. Such findings led to the conclusion that the kinetics of the doping/undoping processes in MEH–PPV represents a crucial factor in determining the emissive properties of the conjugated polymer. A comparison of the ECL emission from analogous polymeric systems, namely poly[2,5‐bis‐(triethoxymethoxy)‐1,4‐phenylene vinylene] (BTEM–PPV) and poly[2,3‐dibutoxy‐1,4‐phenylene vinylene] (DB–PPV), is also reported.     

Impedance Spectroscopy of π‐Conjugated Organic Materials

AC electrical properties of organic light‐emitting diodes with poly(2‐methoxy‐5‐(2'‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV), poly[2,5‐bia(dimethyloctylsilyl)‐1,4‐phenylene‐vinylene] (BDMOS‐PPV), and tris‐(8‐hydroxyquinolate)‐aluminum (AlQ₃) as light‐emitting materials are studied. The frequency‐dependent real and imaginary parts of impedance were fitted using an equivalent circuit. We found that the conduction mechanism is a space‐charge limited current with exponential trap distribution.     

Influence of the Relative Humidity on the Performance of Polymer/TiO2 Photovoltaic Cells

Hydrolysis of titanium(IV ) isopropoxide (TTIP) is a well‐known method for the fabrication of TiO₂. Normally it is made via a sol–gel reaction in the presence of water. In this paper we report on the preparation of flat TiO₂ films for conjugated polymer/TiO₂ photovoltaic cells, from a TTIP/isopropanol solution. It is shown that the morphological structure of the TiO₂ film is strongly dependent on the relative humidity during spin‐coating of the TTIP/isopropanol solution. In bilayer devices consisting of TiO₂/poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV), a low relative humidity (< 25 %, room temperature) is needed in order to form smooth, transparent TiO₂ films. Increasing the relative humidity results in porous TiO₂ films with a high surface roughness, which leads to shunted devices. Apart from bilayer devices, bulk‐heterojunction (BHJ) hybrid TiO₂:MDMO‐PPV photovoltaic cells have been made, by spin‐coating a mixture of TTIP and MDMO‐PPV in toluene. Again a strong relation was found between the relative humidity during spin‐coating and the current–voltage characteristics of the devices. However, in contrast to the bilayer devices, the best BHJ devices were made at higher relative humidity. The observed performance dependence on relative humidity is discussed in relation to the TiO₂ morphology.     

The Effect of Annealing on the Charge‐Carrier Dynamics in a Polymer/Polymer Bulk Heterojunction for Photovoltaic Applications

The effect of annealing blends of poly(2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene) (MDMO‐PPV) and a poly(cyanoether phenylenevinylene) (PCNEPV) on the photoconductivity is studied. Charge carriers are generated by pulsed‐laser excitation and their mobility and decay kinetics are monitored using time‐resolved microwave conductivity (TRMC) measurements. Photoexcitation leads to the formation of an exciton, which can undergo charge separation at an interface between the electron‐donating MDMO‐PPV and the electron‐accepting PCNEPV. The electrons and holes formed in this way must escape from each other to contribute to the photoconductivity. The photoconductivity of the blends is found to increase by almost two orders of magnitude upon thermal annealing for three hours at 100 °C. This increase is attributed to the occurrence of phase separation in the polymer/polymer film, resulting in PCNEPV‐rich parts. The formation of PCNEPV‐rich parts allows the electron to diffuse away from the interface, which favors escape from geminate recombination, leading to a higher photoconductivity.     

Crystallization‐Induced Phase Separation in Solution‐Processed Small Molecule Bulk Heterojunction Organic Solar Cells

The driving forces and processes associated with the development of phase separation upon thermal annealing are investigated in solution‐processed small molecule bulk heterojunction (BHJ) organic solar cells utilizing a diketopyrrolopyrrole‐based donor molecule and a fullerene acceptor (PCBM). In‐situ thermal annealing X‐ray scattering is used to monitor the development of thin film crystallization and phase separation and reveals that the development of blend phase separation strongly correlates with the nucleation of donor crystallites. Additionally, these morphological changes lead to dramatic increases in blend electron mobility and solar cell figures of merit. These results indicate that donor crystallization is the driving force for blend phase separation. It is hypothesized that donor crystallization from an as‐cast homogeneous donor:acceptor blend simultaneously produces donor‐rich domains, consisting largely of donor crystallites, and acceptor‐rich domains, formed from previously mixed regions of the film that have been enriched with acceptor during donor crystallization. Control of donor crystallization in solution‐processed small molecule BHJ solar cells employing PCBM is thus emphasized as an important strategy for the engineering of the nanoscale phase separated, bicontinuous morphology necessary for the fabrication of efficient BHJ photovoltaic devices.