Bulk Heterojunction Organic Solar Cells Based on Crosslinked Polymer Donor Networks

Ph.DDOCTOR OF PHILOSOPH

On the influence of physical and chemical structure on charge transport in disordered semiconducting materials and devices

Achieving fast charge carrier transport in disordered organic semiconductors is of great importance for the development of organic electronic devices. Disordered organic materials generally show low charge carrier mobilities due to their inherent energetic and configurational disorder, and the presence of chemical and physical defects. Efforts to improve mobility typically involve chemical design and materials processing to control macromolecular conformation and/or induce greater crystalline or liquid crystalline order. Whilst in many cases fruitful, these approaches have not always translated into higher bulk mobilities in devices. Addressing the adverse effect on mobility of specific types of disorder or specific defects has proven difficult due to problems distinguishing the many such features spectroscopically and controlling their formation in isolation. In the three experimental Chapters following, we attempt to make clear links between the charge carrier mobility and the presence of specific structural defects or sources of energetic or configurational disorder. In the first experimental study, we investigate hole transport in a family of polyfluorenes based on poly(9,9-dioctylfluorene) (PFO). By controlling the phase formation of the materials through processing and by virtue of their chemical design, we examine the effect on transport of distinct material phases. Remarkably, we are able to isolate the effect of the single chain conformation of PFO known as the beta-phase and show that when embedded in a glassy PFO matrix it acts as a strong hole trap, reducing the mobility of the bulk material by over two orders of magnitude. By fabricating a device with negligible beta-phase, we demonstrate the highest time-of-flight mobility in PFO to date, at over 3 10-2 cm2/Vs. This study provides the first clear and unambiguous example of the effect on transport of a distinct conformational defect in a conjugated polymer. We also demonstrate the adverse effect on mobility of crystallinity in the polyfluorenes. We suggest that our findings may generalise to other systems in the sense that the mobility may be limited by a minority population of structural traps, which may include highly ordered, crystalline regions. Significant mobility improvements may then be more easily achieved by removing the minority ordered phases than by increasing their concentration. We believe that this approach offers an alternative paradigm by which higher mobilities may be obtained in general, and in particular in systems where crystallinity is undesirable. In the second experimental study, we study charge transport in the fullerene derivatives [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), bis-PCBM and tris-PCBM. The fullerene multi-adducts bis-PCBM and tris-PCBM are of interest as alternative OPV acceptor materials with the potential to increase open-circuit voltage. However, most OPV blends employing the multi-adducts have failed to improve upon those employing PCBM. This is thought to be a result of the inferior electron transport properties of the multi-adducts, due to either (i) higher energetic disorder in the multiadducts due to the presence of isomers with varying LUMO energies or (ii) higher con gurational disorder due to a lower degree of order in molecular packing in the multi-adducts than in PCBM. We distinguish the e ects of energetic and con gurational disorder using temperature-dependent ToF and FET measurements. We find that differences in configurational disorder appear negligible, and that the reduced mobility in the multi-adducts is due predominantly to the energetic disorder resulting from the presence of a mixture of isomers with varying LUMO energies. In the third and final experimental study, we examine the charge transport properties of polymer: PCBM blends for OPV, focusing on the PTB7:PCBM and P3HT:PCBM systems. In particular, we address the question of why state-of-the-art OPV systems such as PTB7:PCBM perform so much worse at large active layer thicknesses than P3HT:PCBM. We find that low electron mobility is the main cause of this di erence. The electron mobility in PTB7:PCBM blends, at 10-5 { 10-4 cm2/Vs, is 1-2 orders of magnitude lower than the electron mobility in annealed P3HT:PCBM, at over 10-3 cm2/Vs. The hole mobility, in contrast, is the same to within a factor of approximately three. We hypothesise that the low tendency of PTB7 to order leads to a low degree of phase separation in the blend and to a poorly connected, disordered PCBM phase. We find that increasing the PCBM fraction is very effective in improving electron transport and electrical Fill Factor, but strongly reduces absorption. We suggest that a key challenge for OPV researchers is thus to achieve better connectivity and ordering in the fullerene phase in blends without relying on either (i) a large excess of fullerene or (ii) strong crystallisation of the polymer

Doctor of Philosophy

dissertationSinglet fission (SF) is a process that occurs in some organic semiconductors whereby the photoexcited singlet exciton (SE) undergoes internal conversion to a multiexciton triplet-triplet (TT) state, which subsequently splits into two independent triplet excitons. This process was first observed in crystalline acenes (most notably pentacene) in the 1960s. Renewed interest on singlet fission has been seen dramatically increased in recent years because of its potential in harvesting charges from the triplet excitons in organic photovoltaic cells, thereby doubling the photocurrents. It was shown that the cell external quantum efficiency may exceed 100%, and thus it could potentially overcome the Shockley-Queisser PV efficiency limit under the sun illumination. In this work, we used various optical techniques in our research arsenal to uncover the intrachain singlet fission in a new class of OPV materials, namely low bandgap pi-conjugated polymers, which was used as the electron donor in bulk hetero-junction solar cells. These copolymers produced a record high power conversion efficiency of ~ 8% in an optimum OPV device. Particularly, we introduced two new novel techniques, the nanosecond to millisecond transient photo-induced absorption and transient magneto-photoinduced absorption, dubbed t-PA and t-MPA, respectively, to unravel the population exchange between the singlet exciton and triplet pair (TT) state, which is a new quantum state constituted by two correlated triplet excitons. Using the t-PA in picosecond time domain, we detected the TT state that appears simultaneously with the singlet exciton SE within 300fs time resolution of our experimental setup. The picosecond t-MPA technique further elucidates the nature of TT state, showing its coupling to the SE through their spin exchange interaction with the interaction strength as large as ~30mT. Using the t-MPA together with the ns t-PA, we found that the TT state later separates into two uncorrelated triplets in microsecond time domain. In the copolymers/PC71BM blend, which was used as the active layer in OPV devices, the TT state dissociates, by the unique spin conserved process, into one polaron pair in triplet configuration, PP_T ; leaving behind one triplet on the copolymer chains within 20ps. The PP_T could either dissociate into free charges to generate photocurrents in cell devices or recombine back to triplet excitons. Here we observed the “back reaction”, PP_T --> triplets, in nanosecond time regime, which we identify as a loss mechanism for charge photogeneration in solar cell devices. We also introduce a method to reduce the carrier loss mechanism by the “back reaction” of PP into triplet excitons on the copolymer chains, by adding spin ½ radicals; this method may be especially suitable for copolymer-based OPV cells

Investigation of the Optical Properties of Novel Organic Macromolecules for Solar Cell Applications.

The search for renewable energy sources to replace fossil fuel has been a major research focus in the energy sector. The sun, with its vast amount of energy, remains the most abundant and ubiquitous energy source that far exceeds the world energy demand. The ability to effectively capture and convert energy from the sun in the form of photons will be the key to its effective utilization. Organic macromolecules have tremendous potentials to replace and out-perform existing materials, due to their low-cost, ease of tunability, high absorption coefficient and “green” nature. In this dissertation, spectroscopic techniques of steady state absorption and time-resolved fluorescence spectroscopy were used to show the improved absorption of the oligothiophene-functionalized ZnPc through ultrafast energy transfer. ZnPc is known for its chemical and thermal stability. The power conversion efficiency (PCE) in ZnPc-based solar devices is however, very low because of the poor absorption of ZnPc in the 300 – 550 nm region of the solar spectrum. Oligothiophenes have good absorption in the spectral region where the absorption of ZnPc is poor. Other groups of organic compounds that have gained prominence in the study for the design of efficient active materials for photovoltaic cells are the polymers. In the dissertation, different factors which can affect the performance of organic polymers in photovoltaics systems were investigated and analyzed. The effects of the alteration of conjugation, donor-acceptor groups, heteroatoms and alkyl side chains on the photophysical properties and ultimately the performance of organic polymers in organic photovoltaics were investigated. The different effects were investigated using ultrafast spectroscopic techniques which are capable of providing insight of fluorescence decay dynamics at very short times in a time scale of femtosecond. The electronic structure calculations of the polymers were carried out to provide further evidence to the experimental findings. PTB7, which has one of the best power conversion efficiency in organic photovoltaics, was one of the investigated polymers. Other novel organic polymers based on thiophene and furan framework were also considered.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111532/1/gokesea_1.pd

Ladungsträgerdiffusion und Ladungsträgerübertragungsmechanismus in hybriden bleihalogenidbasierten Perowskit-Materialien

Lead-based halide perovskite solar cells are a young but highly promising research field for future photovoltaic technologies. They have rapidly become the center of attention due to their outstanding power conversion efficiencies, yet the devices’ stability under working conditions is challenging. Furthermore, as a young and unique solar cell concept, their working principle is still under heavy debate. In light of this, the aim of the work at hand is the investigation of lead-based halide perovskite’s ionic conductivity with respect to device stability, as well as charge carrier injection and recombination processes across the interfaces of perovskite-materials with either a hole or an electron transporting layer. To this end, we have investigated model systems of perovskite solar cells using electrochemical methods, i.e. voltammetry, amperometry, and impedance spectroscopy, as well as femto-second transient absorption spectroscopy. We compared these results to figures-of-merit derived from current density voltage characteristics of full devices. We found that the ionic conductivity of hybrid perovskite was dependent on two diffusing species, namely halides and organic cations. This is in contrast to inorganic perovskite, where only halides diffuse, and forceful displacement of its second, non-lead inorganic cation causes decomposition. In ternary blends composed of perovskite nanoparticles, a polymer-based photon absorber, and a fullerene derivative, the charge carrier transport is facilitated by a cascading energy level alignment. We found that the presence of perovskite nanoparticles enables fast and longer-lasting charge carrier separation. In the next step, the electron injection from perovskite into electron transporting materials of different crystallinity was studied. Here, a layer of a polycrystalline fullerene derivative was compared to its amorphous standard analog. The augmented surface area and discrete energy levels of the polycrystalline film was found to offer ultrafast electron injection from perovskite. Finally, we turned towards charge carrier transfer processes occurring across the interface of perovskite with a small molecule type and polymer-based hole transporting materials. In both cases we found comparable injection time frames that corroborate ultrafast injection from perovskite. Furthermore, our study revealed an electron loss-pathway that elucidates how current loss in polymer-based hole transporting materials is correlated to the energetically deep lowest occupied molecular orbital. The findings of the work at hand help to open pathways for a better device stability as well as improved charge carrier injection for higher power conversion efficiencies of future perovskite solar cells

Optical probes of free charge generation in organic photovoltaics

Organic photovoltaics (OPVs) show considerable promise as a source of low cost solar energy. Improving our understanding of the processes governing free charge photogeneration in OPVs may unlock the improvements in efficiency required for their widespread implementation. In particular, how do photogenerated charge pairs overcome their mutual columbic attraction, and what governs the branching between bound and free charge pairs that is observed to occur shortly after their creation? Ultrafast laser techniques such as transient absorption (TA) spectroscopy are the only tools capable of probing the time scales associated with these processes (as short as 10⁻¹⁴ seconds). Challenges include achieving sufficient sensitivity to resolve the tiny signals generated in thin films under solar-equivalent excitation densities, and distinguishing and quantifying overlapping signals due to separate phenomena. We present the development of a versatile and ultra-sensitive broadband TA spectrometer, along with a comprehensive analysis of the noise sources limiting sensitivity. Through the use of referenced shot-to-shot detection and a novel method exploiting highly chirped broadband probe pulses, we are capable of resolving changes in differential transmission < 3 × 10⁻⁶ over pump-probe delays of 10⁻¹³–10⁻⁴ seconds. By comparing the absorption due to photogenerated charges to measurements of open-circuit voltage decay in devices under transient excitation, we show that TA is able to quantify the recombination of freely extractable charge pairs over many decades of pump-probe delay. The dependence of this recombination on excitation density can reveal the relative fraction of bound and free charge pairs. We apply this technique to blends of varying efficiency and find that the measured free charge fraction is correlated with published photocharge yields for these materials. We access a regime at low temperature where thermalized charge pairs are frozen out following the primary charge separation step and recombine monomolecularly via tunneling. The dependence of tunneling rate on distance enabled us to fit recombination dynamics to distributions of recombination rates. We identified populations of charge-transfer states and well-separated charge pairs, the yield of which is strongly correlated with the yield of free charges measured via their intensity dependent recombination. We conclude that populations of free charges are established via long-range charge separation within the thermalization timescale, thus invoking early branching between free and bound charges across an energetic barrier. Subject to assumed values of the electron tunneling attenuation constant, we find critical charge separation distances of ~ 3–4nm in all materials. TA spectroscopy probes the absorption of excited states, with the signal being proportional to the product of population density and absorption cross-section of the absorbing species. We show that the dependence of signal on probe pulse intensity can decouple these parameters, and apply a numerical model to determine the time-dependent absorption cross-section of photogenerated excitons in thin films of semiconducting polymers. Collectively, this thesis presents spectroscopic tools and applications thereof that illuminate the process of free charge generation in organic photovoltaics

에너지변환을 위한 도너-억셉터 시스템 내에서의 여기상태의 다이나믹스

학위논문 (박사)-- 서울대학교 대학원 : 화학부 고분자화학 전공, 2016. 2. Frédéric Laquai.현 논문은 ultrafast time-resolved optical spectroscopy를 이용하여 에너지 변환 도너-억셉터 시스템 내의 excited-state dynamics를 다루었다. 현 연구는 bulk-heterojunction morphology를 구성하는 도너 역할의 diketopyrrolopyrrole-based (DPP) low-bandgap copolymer와 억셉터 역할의 fullerene이 혼합된 유기태양전지의 photophysics를 보고하고자 한다. 두 번째 파트는 long-lived charge-separated states을 형성하기 위해 집광성 물질의 porphyrins, 전자받개 quinones과 전자주개 ferrocenes을 사용한 인공 광합성 반응을 주로 다루었다. Time-resolved photoluminescence spectroscopy 와 transient absorption spectroscopy를 이용하여, low-bandgap polymers PTDPP-TT와 PFDPP-TT 내에서 singlet-exciton의 lifetimes이 20ps 미만임을 밝히고, 폴리머와 PC71BM의 blend 상에서 bound charge-transfer states의 geminate recombination이 주요한 loss channel 임을 보고하였다. 또한, polymers triplet states내로 자유전하의 빠른 non-geminate recombination이 두 blend system에서 모두 관찰되었다. PDPP5T:PC71BM 도너-억셉터 시스템 내에서, 높은 끓는 점을 가지는 두 용액 (i.e. ortho-dichlorobenzene (o-DCB))을 사용한 polymer:fullerene blend는 active layer의 모폴로지를 급격하게 변화시키고 태양전지의 효율을 증가시킨다. 이는 친밀히 섞인 도너/억셉터 물질과 전극으로의 뛰어난 percolation으로 인하여 용이한 전하 생성과 extraction을 야기하기 때문이다. Polymer triplet state이 triplet charge-transfer states로부터 형성된 것과 같이 PDPP5T:PC71BM blend system에서 빠른 triplet-state 형성이 관찰되었다. Multivariate curve resolution (MCR) 분석은 전하의 non-geminate recombination이 폴리머 triplet-state과 밀접하게 의존하고 있음을 보여주었다. 인공적인 광합성 반응시, transient absorption spectroscopy은 photoinduced charge transfer이 quinone-porphyrin-ferrocene (Q-P-Fc) triads에서와 같이 quinone-porphyrin (Q-P)와 porphyrin-ferrocene (P-Fc) diads내에서 매우 효과적임을 증명하였다. 그러나 P-Fc 와 Q-P-Fc system내에서 전하분리상태는 각각의 porphyrin triplet state와 재결합함을 보여주었다. 현 논문은 ultrafast time-resolved optical spectroscopy를 이용하여 에너지 변환 도너-억셉터 시스템 내의 excited-state dynamics를 다루었다. 현 연구는 bulk-heterojunction morphology를 구성하는 도너 역할의 diketopyrrolopyrrole-based (DPP) low-bandgap copolymer와 억셉터 역할의 fullerene이 혼합된 유기태양전지의 photophysics를 보고하고자 한다. 두 번째 파트는 long-lived charge-separated states을 형성하기 위해 집광성 물질의 porphyrins, 전자받개 quinones과 전자주개 ferrocenes을 사용한 인공 광합성 반응을 주로 다루었다. Time-resolved photoluminescence spectroscopy 와 transient absorption spectroscopy를 이용하여, low-bandgap polymers PTDPP-TT와 PFDPP-TT 내에서 singlet-exciton의 lifetimes이 20ps 미만임을 밝히고, 폴리머와 PC71BM의 blend 상에서 bound charge-transfer states의 geminate recombination이 주요한 loss channel 임을 보고하였다. 또한, polymers triplet states내로 자유전하의 빠른 non-geminate recombination이 두 blend system에서 모두 관찰되었다. PDPP5T:PC71BM 도너-억셉터 시스템 내에서, 높은 끓는 점을 가지는 두 용액 (i.e. ortho-dichlorobenzene (o-DCB))을 사용한 polymer:fullerene blend는 active layer의 모폴로지를 급격하게 변화시키고 태양전지의 효율을 증가시킨다. 이는 친밀히 섞인 도너/억셉터 물질과 전극으로의 뛰어난 percolation으로 인하여 용이한 전하 생성과 extraction을 야기하기 때문이다. Polymer triplet state이 triplet charge-transfer states로부터 형성된 것과 같이 PDPP5T:PC71BM blend system에서 빠른 triplet-state 형성이 관찰되었다. Multivariate curve resolution (MCR) 분석은 전하의 non-geminate recombination이 폴리머 triplet-state과 밀접하게 의존하고 있음을 보여주었다. 인공적인 광합성 반응시, transient absorption spectroscopy은 photoinduced charge transfer이 quinone-porphyrin-ferrocene (Q-P-Fc) triads에서와 같이 quinone-porphyrin (Q-P)와 porphyrin-ferrocene (P-Fc) diads내에서 매우 효과적임을 증명하였다. 그러나 P-Fc 와 Q-P-Fc system내에서 전하분리상태는 각각의 porphyrin triplet state와 재결합함을 보여주었다.This thesis covers the investigation of excited-state dynamics in donor-acceptor systems for energy conversion by means of ultrafast time-resolved optical spectroscopy. The main part of this work focuses on the photophysics of organic solar cells consisting of diketopyrrolopyrrole-based (DPP) low-bandgap copolymers as electron donors blended with fullerenes as electron acceptors in a bulk-heterojunction morphology. A second part is dedicated to the study of artificial primary photosynthetic reaction centers based on porphyrins, quinones and ferrocenes as light harvesting, electron accepting, and electron donating moiety, respectively, with the aim to create long-lived charge-separated states. Time-resolved photoluminescence spectroscopy and transient absorption spectroscopy revealed that singlet-exciton lifetimes in the low-bandgap polymers PTDPP-TT and PFDPP-TT are short (< 20 ps) and that in blends of the polymers with PC71BM geminate recombination of bound charge-transfer states is a major loss channel. In addition, fast non-geminate recombination of free charges into the polymers triplet states was observed in both blend systems. For the PDPP5T:PC71BM donor-acceptor system it was found that processing the polymer:fullerene blend with a high-boiling point co-solvent, i.e. ortho-dichlorobenzene (o-DCB), drastically changes active layer morphology and increases solar cell performance, due to more intimately mixed donor and acceptor materials and pronounced percolation pathways to the electrodes, facilitating charge carrier generation and extraction. Fast triplet-state formation was observed in both of the PDPP5T:PC71BM blend systems, as the polymer triplet state can be populated from triplet charge-transfer states. Multivariate curve resolution (MCR) analysis showed a strong fluence dependence pointing to non-geminate recombination of charges into the polymer triplet state. On the artificial primary photosynthetic reaction centers transient absorption spectroscopy verified that photoinduced charge transfer is efficient in quinone-porphyrin (Q-P) and porphyrin-ferrocene (P-Fc) diads as well as in quinone-porphyrin-ferrocene (Q-P-Fc) triads. However, it was also shown that in the P-Fc and Q-P-Fc systems the charge-separated states recombine into the respective porphyrin triplet state. The charge-separated state in the Q-P diad could significantly be stabilized upon the addition of a Lewis acid.1 introduction 1 2 theoretical background 5 2.1 Organic semiconductors 5 2.2 Absorption and emission of electromagnetic radiation by molecular systems 6 2.2.1 Absorption, induced and spontaneous emission - Einstein coefficients 6 2.2.2 Selection rules and Franck-Condon Principle 9 2.2.3 Experimental aspects of absorption 12 2.3 Radiative and non-radiative transitions 13 2.3.1 Internal conversion 14 2.3.2 Fluorescence 15 2.3.3 Intersystem crossing 16 2.3.4 Phosphorescence 16 2.4 Excitons and energy transfer 17 2.4.1 Exciton models 17 2.4.2 Frster energy transfer 19 2.4.3 Dexter energy transfer 21 2.5 Organic solar cells 22 2.5.1 Working principle 22 2.5.2 Architecture of organic solar cells 23 2.5.3 Schottky contact versus Ohmic contact 25 2.5.4 Characterization of organic solar cells 25 2.6 Photophysical processes in organic solar cells 30 2.6.1 Charge transfer and charge dissociation 30 2.6.2 Charge carrier mobility 35 2.6.3 Loss channels in organic solar cells 37 2.7 Materials for organic solar cells 40 2.7.1 Donor materials 41 2.7.2 Acceptor materials 42 2.7.3 Low bandgap polymers 44 2.7.4 Design strategies for low-bandgap polymers 47 2.7.5 DPP-based donor-acceptor copolymers 49 3 experimental methods 55 3.1 Solar cell preparation 55 3.2 Solar cell characterization 57 3.2.1 Current-voltage characteristics 57 3.2.2 External quantum efficiency measurements 57 3.3 Steady state UV-vis absorption spectroscopy 57 3.4 Quasi steady-state photoinduced absorption spectroscopy 57 3.5 Transient absorption spectroscopy 59 3.5.1 Signals in transient absorption spectroscopy 59 3.5.2 Experimental setup 61 3.5.3 Multivariate curve resolution 62 3.6 Time-resolved photoluminescence spectroscopy 64 3.7 Time-of-flight technique 65 4 triplet state formation in dpp-type copolymer/PC71BM blends 69 5 photophysics of PDPP5T:PC71BM solar cells 85 5.1 Introduction 85 5.2 Photovoltaic Performance 88 5.3 Surface Morphology of PDPP5T:PC71BM blend films 90 5.4 PCBM Exciton Dynamics in PDPP5T:PC71BM blend films 92 5.5 Exciton Dynamics in pristine PDPP5T films 94 5.6 Charge- and triplet-induced absorption spectra of PDPP5T 98 5.6.1 Oxidation Study on PDPP5T 98 5.6.2 Triplet-state dynamics in a PDPP5T thin film doped with palladium-tetraanthraporphyrin 100 5.7 Charge generation and triplet-state formation in PDPP5T:PC71BM 102 5.7.1 Charge generation and triplet-state formation on the sub-ns timescale 102 5.7.2 Triplet-state formation by non-geminate recombination of charges on the sub-ns timescale 109 5.8 Charge and triplet exciton recombination on the ns-timescale 114 5.9 Charge Carrier Mobility in the PDPP5T Polymer 118 5.9.1 Field dependence of the hole mobility in pristine PDPP5T determined by the time-of-flight technique 118 5.9.2 Influence of the annealing temperature on the field-effect mobility in pristine PDPP5T 121 5.10 Conclusions 124 6 photoinduced electron transfer in anthraquinoneporphyrin-ferrocens 127 7 modular porphyrin-ferrocene and porphyrin-ferroceneporphyrin 147 8 discussion 163 8.1 Factors limiting the device efficiency in PTDPP-TT and PFDPP-TT based solar cells 163 8.2 Morphology and performance of PDPP5T:PC71BM devices 165 8.3 Triplet state formation in low-bandgap/fullerene photovoltaic blends 168 bibliography 171Docto

Polymer Solar Cells—Interfacial Processes Related to Performance Issues

Harnessing solar energy with solar cells based on organic materials (in particular polymeric solar cells) is an attractive alternative to silicon-based solar cells due to the advantages of lower weight, flexibility, lower manufacturing costs, easier integration with other products, low environmental impact during manufacturing and operations and short energy payback times. However, even with the latest efficiencies reported up to 17%, the reproducibility of these efficiencies is not up to par, with a significant variation in the efficiencies reported across the literature. Since these devices are based on ultrathin multilayer organic films, interfaces play a major role in their operation and performance. This review gives a concise account of the major interfacial issues that are responsible for influencing the device performance, with emphasis on their physical mechanisms. After an introduction to the basic principles of polymeric solar cells, it briefly discusses charge generation and recombination occurring at the donor-acceptor bulk heterojunction interface. It then discusses interfacial morphology for the active layer and how it affects the performance and stability of these devices. Next, the formation of injection and extraction barriers and their role in the device performance is discussed. Finally, it addresses the most common approaches to change these barriers for improving the solar cell efficiency, including the use of interface dipoles. These issues are interrelated to each other and give a clear and concise understanding of the problem of the underperformance due to interfacial phenomena occurring within the device. This review not only discusses some of the implemented approaches that have been adopted in order to address these problems, but also highlights interfacial issues that are yet to be fully understood in organic solar cells