For the last two decades, polymer solar cells (PSCs) have been a cynosure of the photovoltaic community, as evidenced by the growing number of patent applications and scientific publications. Efforts to achieve high power conversion efficiency in PSC, propelled by advances in device architecture, material combination, and nanomorphology control, evolved into poly(3-hexylthiophene-2,5-diyl) (P3HT):phenyl-C61-Butyric-Acid-Methyl Ester (PCBM) bulk heterojunction PSCs, which had been the best seller in PSC research for a decade. Subsequently, PSC research was redirected towards the synthesis of low bandgap materials and optimization of tandem cells, which led to a power conversion efficiency of ∼13%. Even though this efficiency may not be sufficient enough to compete with that of inorganic solar cells, unique properties of PSCs, such as mass roll-to-roll production capability, as well as flexibility and lightness, suggest their niche market opportunities. In this review, an overview of developments in PSCs is presented during the last three decades encompassing pre- and post-P3HT:PCBM era. Emphasis is given on evolution in device architecture, coupled with material selection for pre-P3HT:PCBM era, and synthesis of low-bandgap materials, coupled with a tandem structure for post-P3HT:PCBM era. Last but not least, efforts toward the longer operational lifetime of PSCs by encapsulation are reviewed.

In an effort to forage renewable and affordable energy sources, triggered by depletion of fossil fuels and concerns regarding global warming, harvesting energy from sunlight by using photovoltaic (PV) technology has attracted a surge of interest. PV cells are traditionally classified into three generations, and organic photovoltaics (OPVs) are classified as an emerging PV technology that belongs to the third generation PV cells. The intrinsic properties of organic photovoltaics have suggested them not only as promising alternatives but also as tantalizing complements to conventional inorganic photovoltaics (IPVs). Among various solar technologies, organic photovoltaic technology distinguishes itself as an economically feasible solution owing to its mass production capability by means of low-cost roll-to-roll manufacturing1 although the high cost of raw materials, like functionalized fullerenes, often seems counter. Furthermore, mechanical flexibility, light weight, and transparency of OPV enable portable, wearable energy sources2 or window tinting applications3 which cannot be realized by inorganic photovoltaic technology. By being sui generis in the sense that it is “compatible and competitive” to its inorganic counterparts, organic photovoltaic technology has been a cynosure in the PV community.

The number of patent applications in the OPV area for the last three decades shows steadily growing interest in the OPV technology. The technology life cycle (TLC) is often used to identify and evaluate the status of a technology development, predominantly correlated with the number of patent applications over time.4–7 In general, the normal trend in technology advancement is at first slow, accelerates, reaches a plateau, and then declines. The S-shaped TLC curve consists of four main stages: emerging, growth, maturity, and saturation. Figure 1 shows the cumulative number of patent applications in the OPV area. To elaborate, the number of patent applications in the area of OPV cells was searched by the Cooperative Patent Classification (CPC) code of OPV cells, i.e., “Y02E 10/549”5 at United States Patent and Trademark Office (USPTO). As shown in the patent trend, the number of OPV-related patent applications during the nascent stage of OPV development showed slow growth. Subsequently, the number of OPV related patent applications gained considerable momentum from 2000 to 2009, followed by stagnation until 2013. This patent trend, projected into the TLC, seemingly indicates that OPV technology is past the emerging and growth stages. A significant drop in the number of patent applications from 2014 is evident, but it would be premature to definitively conclude whether current OPV technology falls in the maturity or saturation stage in the TLC curve at present. Analysis on the trend of patent applications in upcoming years will help to better identify and assess the status of OPV technology advancement.

FIG. 1.

Cumulative number of USPTO patent applications in the area of OPV cells (searched by the CPC code of the OPV cell, i.e., “Y02E 10/549”).

FIG. 1.

Cumulative number of USPTO patent applications in the area of OPV cells (searched by the CPC code of the OPV cell, i.e., “Y02E 10/549”).

Close modal

OPV cells use molecular or polymeric absorbers to convert solar energy into electricity. Depending on the types of organic semiconductors used, OPV cells can be classified into two major classes: small-molecule and polymer solar cells (PSCs). Two key characteristic differences between small molecules and polymers for OPV processing are their solubility and thermal stability. To expatiate, small molecules are thermally more stable but less soluble than polymers, vis-à-vis polymers decompose under excessive heat, and their large molar mass is not suitable for evaporation. Hence, small molecules are mostly deposited by sublimation under high vacuum, while in contrast, polymers are deposited by solution-processing, allowing printing modalities. This review will exclusively focus on polymer solar cells.

For the last two decades, extensive efforts have been devoted to researching PSCs, as evidenced by the number of scientific publications in the area.8 Figure 2 shows the number of scientific publications in the area of PSCs and best record efficiency over time. The number of PSC-related scientific articles published from 1995 to 2016 was searched by the keyword “polymer solar cells” on Institute for Scientific Information's (ISI) Web of Science. As seen in Fig. 2, PSC-related publications are steadily increasing. Notably, the number of publications in the area of PSC doubled every 2–3 years from 1998 to 2012. The best record efficiency data were extracted by cross-examining the best research cell efficiencies published by the National Renewable Energy Laboratory (NREL) and the solar efficiency table published in Progress in Photovoltaics.9–20 Both the record efficiency and the number of publications exhibit similar trends: accelerated growth from 2001 to 2013, which plateaued, subsequently.

FIG. 2.

Number of publications in the area of PSCs (left y-axis) and NREL-certified best record efficiency (right y-axis) from 1995 to 2016.

FIG. 2.

Number of publications in the area of PSCs (left y-axis) and NREL-certified best record efficiency (right y-axis) from 1995 to 2016.

Close modal

In this contribution, we first present an overview of PSC device physics. Then, historical developments in PSCs will be reviewed, in terms of device geometry, combination of materials, and nanomorphology control, followed by plasmonic and tandem PSCs. Last but not least, efforts toward the longer operational lifetime of PSCs by encapsulation are discussed.

In this section, the conducting properties of conjugated polymers are explained and the operational mechanism of PSCs is introduced. Emphasis will be given on analogy and comparison with inorganic, epitomized by silicon, semiconductors and solar cells.

The performance of PV cells is evaluated by two metrics: external quantum efficiency (EQE) and power conversion efficiency (PCE). PCE is calculated from the current density versus voltage (J-V) characteristics of PV cells as shown in Fig. 3. The J-V graph of a PV cell is derived by measuring the current density output under simulated solar light input, over a variable voltage bias. The current density at zero bias is termed the short-circuit current density (Jsc), whereas the voltage at zero current is termed the open-circuit voltage (Voc). The fill factor (FF) is defined as the ratio between the maximum power output of a PV cell (shaded area in Fig. 3) and the product of the short-circuit current density and open-circuit voltage. PCE is calculated as

η=JSC×VOC×FFPin,
(1)

where JSC is the short circuit current density, VOC is the open circuit voltage, FF is the fill factor, and Pin is the total incident irradiance. PCE is measured under standard reporting conditions: 1000 W m−2 irradiance, air mass 1.5 global reference spectrum (AM 1.5G), and 25 °C cell temperature.

FIG. 3.

Current density versus voltage characteristics of a PSC.

FIG. 3.

Current density versus voltage characteristics of a PSC.

Close modal

Pin is typically measured with respect to a reference cell. To minimize the spectral error in the measured JSC of the actual test cell, the reference cell should be chosen so that its spectral response matches that of the test cell as closely as possible. For inorganic photovoltaics (IPVs), it can be made by using the same materials and processing conditions as the test cell. For OPVs, however, it is difficult to make a reference cell by using the same materials and processing conditions due to the low stability of OPV devices. Therefore, a silicon reference cell is used for the measurement of OPV devices, in combination with the KG5 filter to minimize the spectral mismatch. However, if the silicon reference cell is used for the OPV measurement, a deviation in the measured JSC arises due to the mismatch of the spectral response between the silicon reference cell and the OPV test cell. Furthermore, solar simulators do not generate the irradiance that matches with the reference spectrum. To compensate these deviations, the calculated spectral mismatch correction factor is used.21,22 The spectral mismatch correction factor (M) is expressed as

M=λ1λ2ERefλSRλdλλ1λ2ERefλSTλdλλ1λ2ESλSTλdλλ1λ2ESλSRλdλ,
(2)

where ERef(λ) is the reference spectral irradiance, ES(λ) is the source spectral irradiance, SR(λ) is the spectral responsivity of the test cell, and ST(λ) is the spectral responsivity of the test cell, each as a function of wavelength (λ). Shrotriya et al.21 described a calibration protocol for the accurate measurement of OPV devices considering the spectral mismatch correction factor, and the protocol is used as a standardized measurement procedure for OPV devices.

Polymers had been regarded strictly as insulators and extensively used in electronics until the discovery of a new class of polymers 40 years ago: conducting polymers. In 1977, Shirakawa, MacDiarmid, and Heeger discovered that polyacetylene films, when oxidized with chlorine, bromine, or iodine vapor, became 109 times more conductive than they originally were.23 Conjugated polymers, such as polyacetylene, have double bonds separated by single bonds along the carbon-based backbones. This bond alternation opens up the forbidden energy bandgap systemic to semiconductors. Intrinsically, conjugated polymers are insulators or at best weak semiconductors. What makes conjugated polymers conductive is the removal or addition of electrons: electrochemical oxidation or reduction, respectively. This redox chemistry is analogous to doping in inorganic semiconductors. By oxidation of conjugated polymers, delocalized electrons are removed from the highest energy pi-bonding orbital, leaving radical cations in which the charge can move along the polymer chain and also be transferred from one chain to another, thereby enabling conjugated polymers to conduct electricity. Hence, suffice it to say that the conductivity of conjugated polymers can be controlled by the degree of doping. The discovery of conducting polymers initially led to research efforts towards conducting properties of polymers for electric wire application. Later, interest has been shifted towards the semiconducting properties of conjugated polymers. Various polymer materials used in PSCs are shown in Fig. 4.

FIG. 4.

Molecular structures of various donor (top row) and acceptor (bottom row) materials used in PSCs.

FIG. 4.

Molecular structures of various donor (top row) and acceptor (bottom row) materials used in PSCs.

Close modal

One of the most important properties of conjugated polymers for the design of PSCs is the bandgap as it controls their electrical and optical characteristics. The bandgap refers to the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The HOMO level of organic semiconductors is analogous to the valence band maximum of inorganic semiconductors, whereas the LUMO level of polymer semiconductors is analogous to the conduction band minimum of inorganic semiconductors.

Photocurrent generation in OPVs can be described as a four step process: (i) exciton generation by light absorption, (ii) exciton diffusion, (iii) exciton dissociation, and (iv) carrier collection. Figure 5 shows schematic illustration of the photocurrent generation process. Internal quantum efficiency (IQE) is the ratio of the number of charge carriers collected at the electrodes to the number of incident photons of a given energy. Given the photocurrent generation steps, IQE can be expressed as follows:

ηIQE=ηabsηdiffηedηcc,
(3)

where ηabs, ηdiff, ηed, and ηcc are the photon absorption efficiency, the exciton diffusion efficiency, the exciton dissociation efficiency, and the charge collection efficiency, respectively. External quantum efficiency (EQE), the ratio of the number of charge carriers collected at the electrodes to the number of absorbed photons of a given energy, can be expressed by

ηEQE=1Rηabsηdiffηedηcc,
(4)

where R is the reflectivity of the substrate-air interface.

FIG. 5.

Photocurrent generation process in OPVs: (i) exciton generation, (ii) exciton diffusion, (iii) exciton dissociation, and (iv) carrier collection, clockwise from top left.

FIG. 5.

Photocurrent generation process in OPVs: (i) exciton generation, (ii) exciton diffusion, (iii) exciton dissociation, and (iv) carrier collection, clockwise from top left.

Close modal

The fundamental difference between organic and inorganic semiconductors in regard to photocurrent generation in PV cells is their dielectric constants; organic semiconductors typically have a low dielectric constant (ε = 2–4), and thus, upon absorption of sunlight, a Coulombically bound electron-hole pair, known as an exciton, is generated. In contrast, inorganic semiconductors have high dielectric constants (e.g., ε > 10 for silicon, etc.), and hence, free electrons and holes are generated.24 Due to Columbic screening differences, excitons in inorganic semiconductors are highly localized with weak binding energies for dissociation and photocurrent collection, whereas the excitons within an organic semiconductor matrix are delocalized over 10 or more bond lengths, with quite significant binding energies required for their dissociation. In addition, the absorption coefficient and carrier mobility in organic semiconductors play important roles in the design of PSCs. Organic semiconductors have much higher extinction, or absorption, coefficients than inorganic semiconductors (∼10 times), enabling thinner active region layers for equivalent photon capture. For PSCs, only about 300 nm is thick enough to absorb most incident light as opposed to a few microns for silicon solar cells. However, due to the low carrier mobility, and subsequently short diffusion length before recombination, of organic semiconductors, about 100 nm is considered as an optimized thickness for PSCs.8 

Generated excitons are by nature charge neutral and therefore do not drift in an electric field. They will diffuse during their lifetime until they are recombined and/or separated. Their lifetime is in nanoseconds, and the diffusion length is only about 5–20 nm on average.25 If an exciton reaches the interface between a donor and an acceptor, characterized by the large electronegativity differences between them, by diffusion in its lifetime, it will dissociate into free charge carriers; otherwise, it will decay via radiative or non-radiative recombination. Charge carriers in inorganic semiconductors separate when they reach the depletion region. Organic semiconductors require a force larger than the exciton binding energy for exciton dissociation, which is typically 0.3–0.4 eV.24,26 However, the average thermal energy in the system (kT @ 300 K = 25.9 meV) is much lower than that. Instead, exciton dissociation is driven by the offset in LUMO energies between the acceptor and the donor, which becomes an OPV efficiency loss in terms of a reduction of its open circuit voltage. Separated electrons and holes migrate to the cathode and the anode, respectively, driven by the work function difference between the two electrodes. In addition, electron and hole mobilities should be balanced to avoid space charge buildup and recombination. For more detailed physics behind OPV and IPV, interested readers are referred to a comprehensive review written by Gregg and Hanna.24 

Inchoate efforts towards improving PCE in PSCs focused on device geometry for improved charge separation, selection of materials, and process optimization for better light absorption and charge transport. Initial efforts led to the successful bulk heterojunction (BHJ) device geometry, in combination with material combination of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene (MDMO-PPV) and phenyl-C61-Butyric-Acid-Methyl Ester (PCBM). Subsequently, interest was shifted to PSCs using the baseline model of poly(3-hexylthiophene-2,5-diyl) (P3HT) interspersed with PCBM. The P3HT:PCBM had been the “best seller” in PSC research for almost a decade since first reported in 2002.27,28 Through nanomorphology control by process optimization, assisted by advances in high resolution scanning probe microscopy techniques, P3HT:PCBM PSC achieved the best NREL-certified record efficiency of 5.4%.29 

Device geometry of OPVs evolved from a simple single layer to a planar heterojunction (PHJ) and finally to the bulk heterojunction (BHJ). Figure 6 shows the device geometries and schematic band diagrams of these three OPV devices.30 This evolution was to develop an optimal device geometry that can strategically address both short exciton lifetime issues and facilitate efficient charge extraction and transport.

FIG. 6.

Device geometry and schematic band diagram of three different OPV devices: (a) single layer, (b) PHJ, and (c) BHJ OPVs. Reprinted with permission from K. M. Coakley and M. D. McGehee, Chem. Mater. 16, 4533 (2004). Copyright 2004 American Chemical Society.

FIG. 6.

Device geometry and schematic band diagram of three different OPV devices: (a) single layer, (b) PHJ, and (c) BHJ OPVs. Reprinted with permission from K. M. Coakley and M. D. McGehee, Chem. Mater. 16, 4533 (2004). Copyright 2004 American Chemical Society.

Close modal

During the nascent stage of OPV development, single layer OPV cells were developed. A single layer OPV is the simplest form of OPVs in terms of device geometry, where an organic single layer is positioned between two asymmetric contacts. The first OPV was reported by Kallmann and Pope31 in 1958. They fabricated an OPV cell by using a single layer of anthracene crystal sandwiched by two NaCl solutions contacted by silver electrodes. In 1982, Weinberger et al.32 reported a polyacetylene based single layer PSC. Efficiencies of these single layer OPV devices were far less than 1%, due to the intrinsic drawback of exciton lifetime. In single layer OPVs, charge dissociation occurs at the organic-electrode interface. However, excitons rarely reach the organic-electrode interface as they decay before reaching the interface due to their short lifetime, and even though they reach the interface and dissociate into charge carriers, most of the electrons will recombine with holes rather than collected by electrodes. Hence, their photocurrent is significantly limited by the exciton diffusion length.

Tang33 proposed a bilayer or planar heterojunction (PHJ) device geometry in 1986 in an attempt to solve this issue. In the PHJ OPV devices, a second organic semiconductor layer is incorporated between the first organic layer and the cathode. The second organic layer has a lower LUMO level than the first organic layer and thereby accepts electrons. The key feature of a PHJ structure is that the charge dissociation occurs at the donor-acceptor interface, as opposed to the organic-electrode interface. The large energy difference, leading to the strong electron withdrawing potential, must exceed the exciton binding energy. If an exciton reaches the donor-acceptor interface, an electron can transfer to the acceptor semiconductor and a hole can transfer to the donor semiconductor. Subsequently, electrons and holes travel in the opposite direction to be collected by the electrodes. Tang reported a PCE of about 1% under simulated AM2 illumination in his PHJ OPV consisting of copper phthalocyanine (CuPc) and perylene tetracarboxylic derivative,33 and this record stood for a decade.

Even though PHJ device geometry allows for a less lossy mechanism than single layer device geometry in terms of charge transport and collection, the exciton lifetime issue still remained. Only excitons generated within their diffusion length to the donor-acceptor interface can contribute to the photocurrent generation. Therefore, the exciton lifetime issue should be addressed by narrowing the distance between the bulk donor region where excitons are generated by light absorption and the donor-acceptor interface where excitons are dissociated. However, reducing the thickness of the active layer cannot be a solution as it is detrimental to absorption of light.

Major advance in terms of device geometry was made in 1995 by two groups. Yu et al.34 and Halls et al.35 introduced the idea of bulk heterojunction (BHJ) geometry. In BHJ OPV devices, donor and acceptor organic materials are interspersed with each other to extend their interface area throughout the active layer. BHJ effectively reduces the distance between the donor and the donor-acceptor interface, thereby allowing for higher probability of generation of excitons close to the interface and their dissociations into free charge carriers. Ideally, the length scale of the blend is close to the exciton diffusion length, and hence, every photon absorbed in the active layer can potentially contribute to the photocurrent. To fabricate a BHJ, the interpenetrating network of donors and acceptors with a bi-continuous phase separation is formed by dissolving both polymers in the same solvent, followed by casting into a single blended layer. Phase separation can occur while the solvent evaporates and during post-deposition annealing (more later). It is serendipity that the phase separation nanomorphology length scale that Mother Nature provides between the donor and acceptor materials is commensurate to the OPV exciton diffusion length. For small-molecule OPVs, both donor and acceptor molecules are co-evaporated in vacuum. Since its inception, BHJ has been the mainstream of OPV research in terms of device geometry so far.

Figure 7 shows an ideal OPV device geometry.36 This design also allows for a significantly longer optical path length, orthogonal to the charge motion, thus decoupling the optical and electrical constraints. The donor and acceptor phases are interspaced by around the exciton diffusion length so that excitons efficiently reach the D-A interface by diffusion. In addition, charge carriers can transport to the electrodes via the interdigitated and percolated “highways.”36 This geometry enables efficient charge separation; however, it is not easy to obtain in classical polymer mixtures due to the disordered nature of polymers36 and the control of the interface quality.

FIG. 7.

Ideal OPV device geometry. Reprinted with permission from M. C. Scharber and N. S. Sariciftci, Prog. Polym. Sci. 38, 1929 (2013). Copyright 2013 Creative Commons Attribution License.

FIG. 7.

Ideal OPV device geometry. Reprinted with permission from M. C. Scharber and N. S. Sariciftci, Prog. Polym. Sci. 38, 1929 (2013). Copyright 2013 Creative Commons Attribution License.

Close modal

Subsequent to the discovery of conducting polymers, a variety of conjugated polymers, in terms of solubility, stability, and electrical conductivity, have been synthesized in the 1990s. Consequently, various combinations of donor and acceptor materials have been used for PSCs to enhance their PCEs. The most widely used combination for BHJ PSCs is a blend of a semiconducting polymer as a donor and buckminsterfullerene (C60) derivative. Buckminsterfullerene (C60) has been the dominant acceptor used ubiquitously in both small molecule OPVs and PSCs, owing to its deep LUMO level and high electron mobility. Furthermore, pioneering discoveries in 1992–1993 demonstrated its ideal charge separation kinetics in combination with donor polymers, by providing sufficient energetics for exciton dissociation. Sariciftci et al.37 reported ultrafast photoinduced electron transfer from poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) onto fullerenes at the interface upon illumination by observing photoluminescence quenching in a thin layer PHJ of MEH-PPV and C60. Subsequently, Lee et al.38 reported that the steady-state photoconductivity of conjugated polymers increased by several orders of magnitude upon adding C60. PCBM, a soluble derivative of buckminsterfullerene, remains the most popular electron transporter in PSCs.

For donor polymers, poly(phenylene vinylene) (PPV) was widely used from the mid-1990s till the early 2000s. The two representative PPV-based materials are poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) and poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene (MDMO-PPV), and they exhibited similar photovoltaic properties. Numerous studies focused on PPV:PCBM BHJ PSCs not only for achieving higher efficiencies in PSCs but also for the better understanding of phase separation,39,40 carrier mobilities,41 and the origin of the open-circuit voltage.26 

For a decade since it was first reported back in 2002,28 P3HT had received tremendous interest as an attractive replacement to PPV-based materials for PSC research. P3HT has advantages over PPV-based materials in that it has a reduced bandgap and a high hole mobility exceeding 0.1 cm2/Vs, with proper morphology control. The absorption edges of MEH-PPV and MDMO-PPV are around 550 nm, whereas the absorption edge of P3HT is around 650 nm, which matches the sun's maximum photon flux in the range between 650 and 700 nm. P3HT:PCBM has been the baseline model of single PSC research, and remarkable improvement in reported PCEs has been achieved.27 

As described above, the morphology of the active layer in the BHJ structure plays an important role in efficient charge dissociation and transport. The refined morphology effectively widens the interface area and provides a continuous percolation pathway, allowing for higher probability of exciton dissociation and charge transport, respectively. Propelled by the advances in the development of high resolution scanning probe microscopy techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM), extensive investigation on the morphology of active layers within PSCs has been reported and contributed to enhancing PCEs.

1. MDMO-PPV:PCBM

Shaheen et al.42 reported 2.5% efficiency in MDMO-PPV:PCBM PSCs. They investigated the effect of two different solvents: toluene and chlorobenzene (CB). The device with a CB-cast active layer showed enhanced Jsc and a threefold PCE increase compared to that with a toluene-cast active layer. The authors analyzed PCBM domains in surface morphology of these two devices. The PCBM domains in the CB-cast active layer were smaller than those in the toluene-cast active layer, thereby yielding increased charge carrier mobility and facilitating enhanced electron collection. This was further supported by TEM and cross-sectional SEM images that show better mixing of PCBM domains with the blend.42 

van Duren et al.43 presented a comprehensive study on the optimal ratio of MDMO-PPV:PCBM PSCs. Among devices with varying ratios of MDMO-PPV and PCBM dissolved in CB, the maximum efficiency was achieved in 1:4 composition of MDMO-PPV:PCBM. The authors related higher PCBM of the optimal ratio to charge mobility and phase separation. With higher PCBM, both electron and hole mobilities increase. Moreover, phase separation occurs only at higher PCBM, reducing carrier recombination.43 

2. P3HT:PCBM

Research on P3HT:PCBM was spurred by the superior properties and optimal combination of P3HT and PCBM.27 Moreover, having benefited from the understanding of the fundamental device physics with insight into process optimization gained from the research on PPV:PCBM,44 the efficiency of P3HT:PCBM PSCs has significantly improved.

It is difficult to conclude the exact one-size-fits-all ratio that works best for all P3HT:PCBM PSCs because endogenous properties, such as regio-regularity, polydispersity, and molecular weight, are different in all polymers used in experiments, depending on the suppliers and batch. Furthermore, PCEs also depend on the thickness of the active layers. During the early stage of P3HT:PCBM research, Schilinsky et al.28 and Padinger et al.45 studied compositions between 1:2 and 1:3 ratios of P3HT:PCBM and reported 2.8% and 3.5% PCEs, respectively. However, according to subsequent studies on the composition of P3HT:PCBM PSCS, consensus has been made that the optimal ratio of P3HT and PCBM is 1:1–0.8. Chirvase et al.46 showed that the maximum PCE occurs between 1:1 and 1:0.9. Huang et al.47 used the time-of-flight technique to show the balanced mobility of both electrons and holes at the composition of 1:1 weight ratio, which is attributed to the formation of a more-ordered structure in the blend. Li et al.48 and Reyes-Reyes et al.49 reported 4.4% and 4.9% PCEs, respectively, at an optimal ratio of 1:0.8.

Thermal annealing has been known to enhance overall PCEs of P3HT:PCBM PSCs. The main reason behind this enhancement is that annealing improves the morphology of the P3HT:PCBM film. To elaborate, the enhanced crystallinity of P3HT and improved charge carrier mobility upon annealing50 lead to improved PCEs. Annealing temperature should be between the glass transition temperature and the melting point of the polymers, which is 12 °C and 178 °C for P3HT.51 Numerous studies were reported regarding the effect of annealing on the morphology of the P3HT:PCBM film, and most of them were performed with thermal annealing at temperatures from 110 °C to 160 °C for 1–30 min.27 Chirvase et al.46 studied the optimum annealing duration on P3HT:PCBM devices annealed at 130 °C. They observed a red shift in P3HT absorption. The red shift was more dramatic in devices with a longer annealing duration. Moreover, the absorption shoulder at around 620 nm was also pronounced in devices with a longer annealing duration, indicating interchain interaction of P3HT and a higher degree of interchain ordering. Reyes-Reyes et al.49 analyzed the performance characteristics of devices annealed at different temperatures and durations. They obtained the maximum JSC of 11.1 mA/cm2 in the device annealed at 150 °C for 5 min, owing to improved film crystallinity. Li et al.48 demonstrated better improvement in PCE in devices annealed after cathode deposition in comparison to those annealed before cathode deposition. The authors speculated that the cathode acts as a barrier that hinders morphology improvement. They also reported that high roughness and coarse texture shown in the AFM image of the optimally annealed film (110 °C, 10 min) lead to a better contact between the polymer and the cathode and thereby facilitate charge collection. Erb et al.50 reported a more systematic study on the correlation between crystallinity of P3HT:PCBM and its optical properties. They provided a clearer explanation on the effect of annealing; upon annealing, isolated PCBM molecules begin to diffuse into larger aggregates, and P3HT aggregates can be converted into P3HT crystallites in these PCBM-free regions, as shown in Fig. 8. They concluded that enhanced PCE in annealed P3HT:PCBM devices is attributed to better electron transport in the PCBM clusters and enhanced absorption of P3HT crystallites. Subsequently, Ma et al.52 achieved a 5% PCE in P3HT:PCBM PSC annealed at 150 °C for 30 min.

FIG. 8.

Schematic change of P3HT:PCBM films upon annealing. Reprinted with permission from Adv. Func. Mater. 15, 1193 (2005). Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

FIG. 8.

Schematic change of P3HT:PCBM films upon annealing. Reprinted with permission from Adv. Func. Mater. 15, 1193 (2005). Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Close modal

Thermal annealing is a simple method of improving efficiency of PSC, but room-temperature annealing is more feasible for large-area, flexible PSCs.53–57 Room-temperature annealing or “solvent-annealing” is a method to control the growth rate of the active layer, by exposing the solution-processed active layer to solvent vapor. Li et al.53 achieved 4.4% PCE in a P3HT:PCBM cell by using the solvent annealing method. 1,2-Dichlorobenzene (oDCB) was used as a solvent for spin casting of polymers to decelerate solvent evaporation because of its higher boiling point. The authors compared PSCs with different growth rates by varying the solvent evaporation time and showed that slow-grown films show better performance than fast-grown films. By analyzing charge carrier mobility and absorption data, the authors concluded that the PCE improvement is attributed to a high-degree of ordering of the polymer by self-organization. Effectiveness of solvent annealing was supported by Mihailetchi et al.,54 who showed that the hole mobility was improved by 33 times in slow-grown films. Overall, in the body of published literature, the aggregate average PCE efficiency for P3HT:PCBM based OPVs is around 3.5%.27 

In this section, recent developments in PSCs beyond the baseline P3HT:PCBM PSC are introduced, in the order of substitution of each layer in P3HT:PCBM PSC with viable replacements, plasmonic PSCs as an optical approach that is complementary to the synthetic chemistry perspective and tandem PSCs to further improve PCEs.

Efficiency is an important factor for determining the technical viability of PV technologies, but the importance of cost and stability cannot be underestimated for commercialization. Even though P3HT:PCBM has been optimized in terms of PCE, there is still room for improvement in terms of cost and stability. Figure 9 shows the device topology of a P3HT:PCBM baseline model, consisting of glass substrates, indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), the P3HT:PCBM active layer, and the cathode. Table I lists the issues identified in each layer of this model and corresponding replacements proposed to resolve, or at least mitigate, these issues. In this section, we will review the issues identified in each layer of conventional P3HT:PCBM PSCs in terms of efficiency, cost and stability, and research efforts towards overcoming the issues.

FIG. 9.

Standard configuration of the P3HT:PCBM baseline model.

FIG. 9.

Standard configuration of the P3HT:PCBM baseline model.

Close modal
TABLE I.

Issues identified in each layer of baseline P3HT:PCBM BHJ PSCs and suggested replacement.

LayerIssueReplacement
Glass substrate Rigid Flexible substrates 
ITO Brittle, high cost, and limited supply of Indium Carbon-based materials 
 Silver nanowire (Ag NW) 
PEDOT:PSS Aqueous and Acidic Transition metal oxide (TMO) 
  Inverted structure 
  Organic Interlayer 
P3HT Absorption Low bandgap materials 
PC60BM Lower VOC Indene-fullerene adducts 
 Optically inert PC70BM 
LayerIssueReplacement
Glass substrate Rigid Flexible substrates 
ITO Brittle, high cost, and limited supply of Indium Carbon-based materials 
 Silver nanowire (Ag NW) 
PEDOT:PSS Aqueous and Acidic Transition metal oxide (TMO) 
  Inverted structure 
  Organic Interlayer 
P3HT Absorption Low bandgap materials 
PC60BM Lower VOC Indene-fullerene adducts 
 Optically inert PC70BM 

1. Glass substrate

Mostly OPVs in the laboratory scale are usually fabricated on rigid substrates, epitomized by glass, for inexpensive and facile processing. Glass substrates maintain planarity during processing and ease of handling of the device. However, key advantages of OPVs are that they are scalable to large area processing, lightweight, flexible, and suitable for applications such as flexible electronics and wearable energy source, which necessitate flexible substrates. Raw plastic materials for flexible substrates are inexpensive, but ITO-coated flexible substrates are more expensive than ITO glass substrates, due to challenges in deposition of ITO on top of flexible substrates. Table II shows a list of representative flexible plastic substrates commercially available, including polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), and their properties.58 

TABLE II.

Commercially available representative flexible plastic substrates and their properties.

Plastic filmProduct nameMelting point (°C)Glass transition temperature (°C)Shrinkage MD @ 150 °C, 30 min (%)Maximum processing temperature (°C)Thermal coefficient of linear expansion (ppm/°C)
PET Melinex ST506 255 78 0.1 150 20–25 
PEN Teonex Q65FA 263 120 0.05 180–200 18–20 
Polyimide Kapton HN 520 350 0.17 400 20 
Plastic filmProduct nameMelting point (°C)Glass transition temperature (°C)Shrinkage MD @ 150 °C, 30 min (%)Maximum processing temperature (°C)Thermal coefficient of linear expansion (ppm/°C)
PET Melinex ST506 255 78 0.1 150 20–25 
PEN Teonex Q65FA 263 120 0.05 180–200 18–20 
Polyimide Kapton HN 520 350 0.17 400 20 

Flexible substrates are required to be compatible with standard OPV fabrication processes, including spin-coating, thermal annealing, and vacuum thermal evaporation. Therefore, physical, chemical, and thermal stabilities should be considered for the selection of flexible substrates to ensure that they can maintain surface flatness and dimensions during the subsequent processes, be highly resistant to chemicals used during spin-coating of PEDOT:PSS and P3HT:PCBM, and endure thermal stress during ITO deposition, annealing, and evaporation. Furthermore, flexible substrates are expected to have a high refractive index and transmittance. One of the most widely used flexible substrates is biaxially oriented polyethylene terephthalate (BOPET), commercially branded as Mylar and Melinex. Kaltenbrunner et al.59 demonstrated P3HT:PCBM devices fabricated on the Mylar substrate less than 2 μm thick, with its PCE equal to those on glass substrates.

2. ITO replacement

ITO has been the most dominant transparent conducting electrode in OPVs to collect generated holes, owing to its good transparency and conductivity. Issues identified with ITO are its fragility, especially with flexible substrates, high price, toxicity, and limited supply of indium. Therefore, inexpensive ITO alternatives have been sought that are more robust, especially with flexible substrates, and possess transparency and conductivity comparable to ITO. Candidate materials that have been investigated are carbon-based materials, e.g., carbon nanotubes (CNTs) and graphene, and metal nanowire. Among them, CNTs are promising, thanks to their flexibility, thermal and chemical stabilities, and solution processibility.60,61 Moreover, they possess high transmittance over a very broad spectral range, and their work function (4.7–5.2 eV) matches well with ITO. However, the challenge is to optimize the thickness of the CNT layer to compromise the tradeoff between sheet resistance and transparency. The thicker CNT layer reduces sheet resistance, at the risk of transparency. Short circuits are possible by CNTs protruding out of the plane of the film, bisecting the OPV active region.

In earlier work, Pasquier et al.62 reported 1% PCE in the P3HT:PCBM device where single-wall (SW) CNT was used in lieu of ITO. This value was higher than that of a control device where ITO was used as an anode. Rowell et al.63 used a printing method to deposit SWCNT on the PET substrate and achieved 2.5% PCE in P3HT:PCBM devices, which was 80% of the PCE in their conventional ITO-based P3HT:PCBM (3%) control device. It has also been reported that compared to multi-wall (MW) CNTs, oxidized MWCNTs yield nearly three-fold PCEs in P3HT:PCBM devices.64 Moreover, some groups used an acid oxidation treatment to increase the intrinsic conductivity of CNTs by p-doping, coupled with the reduction of the junction resistance.65,66 Barnes et al.67 fabricated OPV devices with SWNTs post-functionalized by acid oxidation treatment, which increased PCE from 3.5% to 4.1% with respect to conventional ITO devices.

Another carbon-based material, graphene, has also been investigated for its potential of replacing ITO as it exhibits many of the positive characteristics of CNTs. Incorporation of pristine graphene as an anode in PSC devices leads to poor performance because the hydrophobic nature of the graphene surface is detrimental to uniform surface coverage of the PEDOT:PSS layer. In this sense, studies on graphene as an ITO replacement mainly focused on modification of its surface wettability without degrading conductivity by doping.68 Wet doping by the acid treatment method using pyrenebutanoic acid succinimidyl ester,69 nitric acid,70 and hydrochloric acid71 has been reported, but overall PCE of devices using graphene-based materials as the anode has not surpassed that of ITO-based devices.

Silver nanowires (Ag NWs)72 were suggested as promising ITO replacements, owing to their excellent properties such as flexibility on flexible substrates, as well as transparency and conductivity even superior to ITO that may compensate the high cost of silver. Moreover, dispersed random networks of Ag NWs improve photocurrent generation by enhanced light scattering in PSCs and the donor-acceptor interface by inherent roughness. Two issues of Ag NWs in PSC devices were identified. Foremost, the lower work function difference between Ag NWs/PEDOT:PSS and the cathode, compared to that between ITO and the cathode, yields lower Voc. This issue has been resolved by using an Ag NW anode in combination with buffer layers. Various cathode buffer layers have been used, including titanium dioxide (TiO2) and cesium carbonate (Cs2CO3), coupled with Ag NWs in P3HT:PCBM devices to demonstrate similar or improved PCEs with respect to ITO-based devices. Another issue is the surface roughness due to the random network of Ag NWs, causing shorting issues in devices and thereby limiting the conductivity. To elaborate, the voids between individual AgNWs in the film also affect the charge extraction properties of the Ag NW electrode particularly when the size of the void is greater than the carrier diffusion length. Some groups used the composite of Ag NWs and PEDOT:PSS instead of ITO/PEDOT:PSS to smoothen the surface.73,74 In a manuscript by Ajuria et al.,75 ZnO was used on top of Ag NWs to fill up the void between Ag NWs. A 3.85% PCE was reported in devices with Ag NW/ZnO as an anode, which was higher than ITO-based devices (3.53%).

3. PEDOT:PSS replacement

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) has been dominantly used as a hole transport layer (HTL) in OPVs. It has a high work function of 5.0 eV and thereby effectively reduces the hole injection barrier. The main function of PEDOT:PSS in OPVs is that it planarizes the rough surface of ITO and therefore prevents any local short-circuiting.76 Furthermore, its high transparency and conductivity are favorable to PSC operation. Therefore, some of the best record efficiencies in PSCs are those using PEDOT:PSS as a HTL. Nevertheless, its acidic nature is detrimental to ITO, as it slowly etches ITO over time and consequently indium diffuses into the active layer.77 Moreover, it is an aqueous solution and hygroscopic,78 and hence, once deposited, it becomes an endogenous source of moisture that is harmful to the lifetime of OPVs.

Most widely studied materials for HTL are transition metal oxides (TMOs) owing to their high work function, notable transparency, and good stability.79 Among them, molybdenum trioxide (MoO3), tungsten trioxide (WO3), and vanadium pentoxide (V2O5) have demonstrated their potential of replacing PEDOT:PSS.80 Ultraviolet photoelectron spectroscopy (UPS) studies showed that MoO3 and WO3 semiconductor films are strong n-type and their conduction band is deeper than the HOMOs of common polymer semiconductors.81,82 Therefore, charge transfer occurs via interfacial electron extraction from the HOMO through the deep-lying conduction band of MoO3.83 Some groups reported solution-processed MoO3 as an HTL.83–86 Liu et al.83 used aqueous MoO3 solution prepared by the hydration method for spin-coating of the MoO3 film. The performance of their solution-processed MoO3 HTLs was comparable to, or even better than, that of PEDOT:PSS and thermally evaporated MoO3. Girotto et al.84 fabricated MoO3 HTL by sol-gel techniques and demonstrated that these solution-processed MoO3 HTLs are equally effective as PEDOT:PSS or thermally evaporated MoO3, for both P3HT:PCBM PSCs and small-molecule OPV cells. Inverted PSC structures have also been investigated in conjunction with TMOs to further enhance device stability. In inverted structures, the anode is located on top of the device stack and the cathode is located on top of the substrate, as opposed to a standard structure (Fig. 10). Device polarity in conventional and inverted structures is irrespective of electrodes and controlled by the relative position of HTL and the electron transport layer (ETL) because the work function of ITO lies between the HOMO and LUMO levels of most PSC materials.87 TMOs are mostly thermally evaporated to control the thickness and circumvent the miscibility issue that may arise when TMOs are solution-processed on top of active layers in inverted PSCs. HTLs and ETLs can also function as optical spacers in regular and inverted PSCs, respectively.88,89 By controlling their thickness, electric field distribution in PSC can be optimized to maximize absorption in active layers. Studies on evaporated MoO3 films used in lieu of PEDOT:PSS in P3HT:PCBM PSCs, ceteris paribus, demonstrated enhanced stability while maintaining PCEs comparable to those with PEDOT:PSS.90,91 P3HT films, grown on the WO3 film, have a higher degree of ordering and hole mobility compared to those grown on PEDOT:PSS.92 Devices with evaporated V2O5 as HTL also exhibited PCB ∼3%, but compared to MoO3 and WO3, research on V2O5 is still rudimentary due to its toxicity and partial absorption overlap with PCBM.79 

Solution-processible p-type organic materials have also been investigated for their potential as PEDOT:PSS replacement. Similar device performances as PEDOT:PSS control devices have been demonstrated by using sulfonated poly(diphenylamine) (SPDPA),93 polyaniline (PANI),94 polyaniline–poly(styrene sulfonate) (PANI–PSS),95 and poly(4-styrenesulfonate)-g-polyaniline (PSS-g-PANI).96 

FIG. 10.

(a) Conventional and (b) inverted structures of PSC.

FIG. 10.

(a) Conventional and (b) inverted structures of PSC.

Close modal

4. P3HT replacement

Figure 11 shows the Earth's sun irradiance as a function of wavelength.97 P3HT has a bandgap of 1.9 eV, and it can possibly harvest only up to 22.4% of the available photons. The poor overlap between the P3HT absorption spectrum and the sun's irradiance spectrum is one of the key limiting factors in achieving high PCEs. In fact, this has become an impetus for targeting PSCs to indoor lighting applications, where the spectral overlap is better matched.98 In an effort to harvest sunlight with wavelengths above the absorption edge of P3HT (650 nm) and thereby improve JSC, low bandgap polymers have been synthesized and used in PSCs. Low bandgap polymers have a lower HOMO and/or higher LUMO level. The lower HOMO level of a donor polymer may result in the loss of VOC because VOC is proportional to the HOMO level of the donor and LUMO of the acceptor. In addition, the higher LUMO level of a donor polymer may result in the loss of electron transfer.97 Since the solar spectrum has its maximum of the photon flux at around 700 nm, the semiconducting polymers with an absorption maximum at around 700 nm would have the highest matching of the spectra with a single absorption band. This approach also enables a relatively high open-circuit voltage (VOC) since a large difference between the HOMO and lowest unoccupied molecular orbital (LUMO) energies can be maintained for the acceptor material (PCBM).

FIG. 11.

Photon flux from the sun (AM 1.5) as a function of wavelength. Integrated photon flux and current are shown on the right y-axis, as a percentage of the total number of photons and as obtainable current density, respectively. Reprinted with permission from Sol. Energy. Mater. Sol. Cells 91, 954 (2007). Copyright 2007 Elsevier B. V.

FIG. 11.

Photon flux from the sun (AM 1.5) as a function of wavelength. Integrated photon flux and current are shown on the right y-axis, as a percentage of the total number of photons and as obtainable current density, respectively. Reprinted with permission from Sol. Energy. Mater. Sol. Cells 91, 954 (2007). Copyright 2007 Elsevier B. V.

Close modal

Low bandgap polymers are synthesized by incorporating an alternating donor and acceptor unit (D-A) in the polymer.99 Intramolecular charge transfer between donor and acceptor units results in a lowering of their bandgap. Low bandgap polymers can be classified by the acceptor units into four categories: benzodiathiazole (BT), thienopyrrolodione (TPD), diketopyrrolopyrrole (DPP), and thienothiophene (TT).100 PCE of 10.2% and JSC of 19.9 mA/cm2 were reported in a PSC employing a BT based low bandgap polymer donor.101 TT based low bandgap polymers also demonstrated highly efficient PSC devices, and PCE of 10.3% was reported with 90% concentration of PCBM.102 A detailed review on PSCs using low bandgap materials is reported elsewhere.100,103

5. PC60BM replacement

PC60BM has long been the material-of-choice for acceptors in PSCs. However, it is optically inert and does not collect sunlight in the visible range due to its soccer-ball symmetry of the C60 molecule, which leads to forbidden lowest energy transition. In comparison with PC60BM, PC70BM exhibits stronger absorption in the visible range, as shown in Fig. 12.104 Therefore, higher JSC can be obtained in PSCs incorporating PC70BM, leading to higher PCE compared to devices with PC60BM.105 However, the very high cost of PC70BM due to its purification process limits its widespread application in PSCs. Another issue with PCBM is that it has a relatively low LUMO level, which is a limiting factor in achieving high Voc, because Voc is proportional to the difference between the LUMO of the acceptor and HOMO of the donor material.26 Efforts toward searching for PC60BM replacement acceptors that exhibit stronger absorption and higher LUMO levels than PC60BM led to synthesis of indene-fullerene adducts. Advantages of indene C60 adducts are more facile synthesis, better solubility in common organic solvents, and stronger visible absorption than PC60BM as shown in Fig. 12. Synthesis of Indene C60 bis-adducts (ICBA) was first reported by He et al.106 The LUMO level of the synthesized ICBA was 0.17 eV higher than that of PCBM. A P3HT:ICBA device showed higher VOC of 0.84 V and higher PCE of 5.44%, compared to VOC of 0.58 V and PCE of 3.88% in the P3HT:PCBM device. Through further optimization by morphology control and addition of a ETL, PCEs of 6.5%107 and 7.5%108 were obtained in P3HT:ICBA devices, respectively. Combining improved absorption of C70 compared to C60 and higher VOC of ICBA compared to PCBM, indene C70 bis-adducts (IC70BA) were also studied. Initially, PCE of P3HT:IC70BA devices reached 5.64%109 but later increased to 7.4% by adding 1-chloronaphthalene as an additive.110 

FIG. 12.

Comparison of absorption spectra of PC60BM, PC70BM, and ICBA. Reprinted with permission from Appl. Phys. Lett. 103, 203301 (2013). Copyright 2013 AIP Publishing.

FIG. 12.

Comparison of absorption spectra of PC60BM, PC70BM, and ICBA. Reprinted with permission from Appl. Phys. Lett. 103, 203301 (2013). Copyright 2013 AIP Publishing.

Close modal

Kang et al.111 systematically synthesized a series of indene-C60 multi-adducts, including indene-C60 mono-adduct (ICMA), indene-C60 bis-adduct (ICBA), and indene-C60 tris-adduct (ICTA), with 1, 2, and 3 indene solubilizing groups, respectively. The addition of indene solubilizing groups lowered LUMO levels, thereby increased VOC of devices: 0.65, 0.83, and 0.92 V for P3HT:ICMA, P3HT:ICBA, and P3HT:ICTA, respectively. Photovoltaic performance of P3HT:ICBA was superior to that of P3HT:PCBM control devices, in terms of VOC, JSC, and PCE (5.26% vs. 3.59%). In spite of its highest VOC, P3HT:ICTA devices exhibited lower PCE than other devices due to the lower fill factor and JSC. Based on the mobility calculation, the authors concluded that poor mobility in ICTA is one of the reasons for the poor performance in P3HT:ICTA devices. This conclusion was further supported and expatiated by Nardes et al.112 

Through synthesis of novel low bandgap materials, coupled with morphology control by process optimization, ∼11% PCE was achieved in single junction PSCs.113 Tandem PSCs, where two or more single junction cells that absorb in different wavelength ranges are stacked, are being explored to overcome the limitations of single junction cells. VOC of the tandem cell is the sum of VOC of the subcells, assuming no potential losses in the interconnection layer, whereas JSC of the tandem cell is limited by the subcell delivering the smallest JSC assuming the same fill factor for the two subcells.114 To circumvent efficiency losses due to JSC limitations in two-terminal tandem cells where subcells are monolithically stacked, three- or four-terminal tandem cells where subcells are connected in parallel were reported.115,116 Also, photocurrent generated by subcells in a tandem cell may differ from that of single cells fabricated under the same conditions, due to the spectral overlap between the two subcells and the change in optical field distribution in the tandem structure.117 Therefore, the appropriate choice of subcells to minimize the absorption overlap and optimization of optical field distribution to maximize absorption in active areas of subcells are crucial.118 

The interconnecting layer (ICL) connecting the two sub-cells is very critical in the fabrication of tandem cells. The interconnecting layer (ICL) should efficiently collect electrons from one sub-cell and holes from the other subcell. Moreover, it should act as an efficient recombination zone without potential loss of the collected electrons and holes. Any barrier to charge carrier recombination will lead to accumulation of carriers in the interlayer, increasing the series resistance of the device. Therefore, it is important that n-type and p-type layers form an Ohmic contact for efficient carrier recombination as well as forming Ohmic contacts with the polymer sub-cells.118 The mostly designed ICLs can be categorized into two major categories: ETL-conducting layer-HTL (E-C-H) and ETL-HTL (E-H) for conventional tandem PSCs and HTL-conducting layer (H-C-E) and HTL-ETL (H-E) for inverted tandem PSCs.118 E-C-H and H-C-E ICLs employ metal conducting layers that serve as a recombination center. Even though high efficiencies have been achieved with these ICLs, E-H or H-E ICLs have been dominantly employed for tandem PSCs, to circumvent thermal evaporation steps to deposit metal and optical losses from the metal layer. Among them, the most widely used E-H ICLs for conventional tandem PSCs are based on the combination of a solution-processible n-type metal oxide, such as TiOx, ZnO, Nb2O5, and high work function PEDOT:PSS.118 Similarly, a combination of solution-processible ZnO and PEDOT:PSS is the most popular ICLs for inverted tandem PSCs.118 

It has been reported that theoretical Jsc of P3HT:PCBM PSCs could be close to 15.2 mA/cm2 (IQE = 100%), and the maximum Jsc is achieved when the thickness of the active layer is 5000 nm.119 However, practically achieved Jsc of actual devices, where the active layer is 100–200 nm thick, remains in the range of 10–12 mA/cm2.119 Efforts toward achieving high JSC in PSCs have mostly been driven by synthesis of low bandgap materials, coupled with PC70BM to harvest more sunlight and corresponding optimization of processing conditions to facilitate charge transport in BHJs.

Plasmonic PSCs have been developed as an alternative approach to attain higher JSC. This optical approach can be generically applied irrelevant to any material combination, including low bandgap polymers, and hence complements the aforementioned chemistry-based approaches.

A major bottleneck in achieving high JSC is the fundamental tradeoff between absorption of light and collection of charge carriers.120 Thickness should be compromised to ensure sufficient light absorption as well as efficient charge collection. Therefore, the key challenge here is to enhance light absorption in an active layer of limited thickness for efficient charge transport. Recently, plasmonic excitation by metal nanoparticles has attracted considerable interest as a promising approach to trap or confine the light inside the active layer. Plasmonic enhancement is achieved through various mechanisms, including far-field scattering, near-field enhancement by localized surface plasmon resonance (LSPR) for metal nanoparticles, and near-field enhancement by surface plasmon polaritons (SPPs), also known as propagating surface plasmons (PSPs) for planar metal surfaces.121 Figure 13 depicts these three mechanisms, respectively. Essentially, plasmonics acts to focus and intensify the light onto the photoactive region by its near-field lensing properties at resonance. To elaborate, far-field scattering increases the optical path length of incident photons by folding the light into the absorbing medium, while LSPR increases the effective absorption cross-section by localizing the incident field. Both far-field scattering and LSPR contribute to the enhanced light absorption and extended light absorption band in PSCs, leading to improved JSC.122 

FIG. 13.

Three different mechanisms of plasmonic enhancement: (a) far-field scattering, (b) near-field enhancement by LSPR, and (c) near-field enhancement by SPP. Reprinted with permission from Nat. Mater. 9, 205 (2010). Copyright 2010 Nature Publishing Group.

FIG. 13.

Three different mechanisms of plasmonic enhancement: (a) far-field scattering, (b) near-field enhancement by LSPR, and (c) near-field enhancement by SPP. Reprinted with permission from Nat. Mater. 9, 205 (2010). Copyright 2010 Nature Publishing Group.

Close modal

Surface plasmon resonance (SPR) can be tuned by the size, shape of metal structures, particle material, and dielectric properties of the surrounding medium.121 A variety of classes of plasmonic nanoparticles have been reported in terms of shape and size of nanostructures, as well as particle materials; gold (Au) and silver (Ag) nanoparticles are the most widely employed materials because their resonances are within the absorption band of most PSCs. For instance, small Ag and Au particles in air have plasmon resonances at 350 nm and 480 nm, respectively, which can be shifted depending on their size and shape.121 Diverse shapes of nanoparticles have been employed, including spheres,123–143 prisms,144–146 rods,147,148 triangle,149 grating,150 and wires.151–154 The size of the nanoparticles is in the range of a few nanometers to micrometers. In addition, insulating layers, such as silica (SiOx) or titania (TiOx) shells, are often used for metal nanoparticles to reduce the charge recombination.

Plasmonic nanoparticles are often incorporated into other layers used in PSCs, e.g., anodes,150,152,155–158 HTLs125,134,146,159–168 and ETLs,126,127,136,137,142,147,169,170 BHJ active layers,123,124,131,135,148,171–180 or cathodes.181 The interaction between surface plasmons and excitons decreases exponentially as the distance between metal nanoparticles and the active layer increases. In this sense, plasmonic nanoparticles embedded inside the BHJ active layer is considered as the best approach for near-field enhancement. However, if the distance between the metal nanoparticles and the active layer is too close, exciton quenching occurs via non-radiative energy transfer.182 Moreover, nanoparticles are usually synthesized by the Brust183 and the Turkevich184 aqueous preparation methods, and hence, hydrophobic ligands need to be attached to the nanoparticles to resolve miscibility issues with organic active layers. To avoid exciton quenching and miscibility issues, some groups embedded nanoparticles in HTLs and ETLs, in which case the incorporation of nanoparticles into PEDOT:PSS HTL is favorable for facile dispersion due to their hydrophilic nature. In this configuration, thicknesses of HTLs and ETLs as well as the size of nanoparticles play an important role in plasmonic enhancement. If nanoparticles are smaller than the thickness of HTLs or ETLs, they are covered with charge transport layers.122 As a consequence, they do not protrude into the active layer and rather affect the electron and hole transfer to the electrodes. Larger nanoparticles interfere with light absorption by inducing increased surface roughness, as well as contributing to back scattering. Plasmonic nanoparticles are also incorporated into the electrodes to facilitate charge carrier extraction, owing to their metallic nature. Some groups incorporated nanoparticles in two layers, e.g., HTL and ETL,133,145 HTL and active layer,140 anode and active layer,185 and ETL and cathode,14 for further plasmonic enhancement.

To circumvent the aforementioned miscibility and recombination issues, plasmonic nanoparticles have also been deposited via solutions to form an independent layer at the interface between two adjacent layers in PSCs, i.e., between the anode and HTL,128–130,149,186–188 or between HTL and the active layer.139,151,182,189–196 Nanoparticles placed between HTL and the active layer often show superior plasmonic enhancement to those placed between the anode and HTL, due to the exponential decay of the interaction between surface plasmons and excitons. However, exciton quenching and surface recombination issues were speculated in this configuration. For the optimized plasmonic nanoparticle layer placed between PEDOT:PSS and the active layer, miscibility issues should be considered because of the hydrophilic nature of PEDOT:PSS and hydrophobic nature of the active layer. Detailed reviews, including the comprehensive list of plasmonic OPVs,122 are reported elsewhere.120–122 

For the last three decades, OPV research has mainly focused on achieving higher efficiencies. Through sophisticated device geometry, tailored development of new polymers benefited from their synthetic flexibility, and heuristic morphology control aided by various nanoscale microscopy technologies, NREL-certified efficiencies of PSCs reached 10.6%197 and 11.5%113 in tandem and single cells, respectively. Even though remarkable improvement has been made in the last three decades, current PCE of PSCs is not comparable to that of inorganic counterparts, e.g., ∼46% in GaInP/GaAs and GaInAsP/GaInAs multijunction solar cells.20 However, unique properties of OPVs, such as large area scalability, synthetic flexibility of organic materials as well as mechanical flexibility of OPV cells, and lightness, may find potential niche market opportunities, particularly for point-of-use applications, such as wearables. For instance, they may be especially useful for domestic applications, such as powering autonomous Internet-of-Things objects, because they are known to perform well under diffuse light98 and they are spectrally matched better to indoor lighting, which is predominately in the visible range, with less infrared components.

In addition to efficiency, cost is another indispensable parameter that governs technical and commercial viabilities of PV technologies. The importance of cost is underlined in applications that do not require high efficiency PV cells. Low-cost processing capability has always been the main thrust and excitement for OPV research, but there are other costs that contribute to cost-of-ownership of completed OPV modules. An OPV cost analysis reported in 2011198 revealed that the cost of raw materials occupies the lion's share in the production of OPV modules, up to 80%. As shown in Fig. 14, most of the material cost is attributed to ITO coated on PET (maximum 51.2% of the total material cost) and the P3HT:PCBM active layer (maximum 27.2% of the total material cost).198 P3HT and PCBM are specialty chemicals, but PCBM is more than 12 times more expensive than P3HT. However, PET, ITO, and PCBM are the predominant flexible substrate, anode, and acceptor materials for PSCs, respectively, and their candidate replacements are yet to demonstrate both superior performance and cost effectiveness over them. Consequently, there is not much room for improvement in the cost of OPVs.

FIG. 14.

Absolute (top) and fractional (bottom) costs of materials used in the manufacture of a 1 m2 OPV module. Reprinted with permission from Energy Environ. Sci., 4, 3741 (2011). Copyright 2011 The Royal Society of Chemistry.

FIG. 14.

Absolute (top) and fractional (bottom) costs of materials used in the manufacture of a 1 m2 OPV module. Reprinted with permission from Energy Environ. Sci., 4, 3741 (2011). Copyright 2011 The Royal Society of Chemistry.

Close modal

Coupled with efficiency and cost, lifetime also plays a penultimate role in potential commercialization of PV technologies. These three parameters are interdependent, and relative importance of each parameter depends on applications. The importance of lifetime is underlined in OPVs because they are known to degrade in ambient air, mostly because of moisture and oxygen. Furthermore, ironically, UV light and high temperature are also known as sources of OPV degradation.199 These sources of degradation and their combination affect each layer and their interfaces in OPVs. Therefore, OPV cells are generally fabricated inside the glove box filled with inert gas, i.e., nitrogen or argon. After fabrication, they are often encapsulated before exposure to ambient air. The most preponderant encapsulation method is by using two barrier films and a sealant; OPV cells are sandwiched between front and back barrier films, whose edges are glued together by the sealant. Therefore, OPV cells can be protected vertically by barrier films and horizontally by the sealant.

Both barrier films and sealants are required to have thermal and light stabilities for outdoor applications, not to mention ultra-low gas permeability. The water vapor transmission rate of 10−6 g m−2 day−1 and the oxygen transmission rate of 10−3 cm3 m−2 day−1 are often used as criteria for gas permeability.200 In addition, flexibility is another requirement for both barrier films and sealants so that they are amenable to roll-to-roll processing, and thereby, OPV cells can retain their cost effectiveness. Moreover, very high transmittance is desirable for front barrier films as they optically couple with OPV cells. Last but not least, thermally curable encapsulants are required to be cured at a temperature lower than the glass transition temperature of flexible substrates and the annealing temperature of bulk heterojunction OPV devices, typically lower than 150 °C. Inorganic barrier films provide high barrier towards moisture permeation, but they are not compatible with roll-to-roll processing. In contrast, polymer barrier films are amenable to roll-to-roll processing, whereas they exhibit poorer gas permeability. Hence, a combination of organic and inorganic materials was used to take advantage of flexibility of organic materials, while retaining low gas permeability in multilayered architecture.201 Various groups used single layer, thermally curable, polymer composites and multilayered films for OPV encapsulation and demonstrated improved lifetime compared to unprotected OPV cells.201 

PSCs have gained considerable interest owing to their unique properties as well as low-cost processing capability. As efforts are directed toward achieving higher PCE, PSCs evolved from single layer to BHJ in terms of device geometry and from PPVs to P3HT for donor materials. Consequently, the baseline model of P3HT:PCBM had long been the best seller in PSCs and optimized to improve efficiency, cost, and lifetime. Focus was shifted to tandem PSCs using wide and low bandgap polymers. Through optimization of subcells in tandem PSCs, assisted by synthesis of low bandgap polymers, PCEs of both single and tandem PSCs climbed over 10%, which was considered the break-even point by the National Renewable Energy Laboratory for commercial viability. As the PCE of PSCs reaches a plateau, the importance of lifetime becomes significant. Relative importance of efficiency, cost, and lifetime of PSCs is dependent upon their applications. Although the current reported PCE of PSCs is not comparable to that of inorganic PV cells, unique properties of PSCs are expected to open up niche market opportunities.

This material is based upon the work supported by the National Science Foundation under Grant No. 1202465.

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