Srinivas Sista - Academia.edu (original) (raw)

Papers by Srinivas Sista

Sid's Digest Of Technical Papers, Jun 1, 2015

The red‐line emission of PFS, K2SiF6:Mn4+, phosphor centered at 631 nm results in phosphor conver... more The red‐line emission of PFS, K2SiF6:Mn4+, phosphor centered at 631 nm results in phosphor converted LED package efficacy improvements of >10% relative to packages using conventional Eu2% doped nitride broad‐band red‐emitting phosphors. Improvements in absorption, quantum efficiency and stability under high humidity and high light flux will be presented that have resulted in commercialization of this material under GE TriGain™ Technology.

Journal of Applied Physics, Nov 21, 2022

Downconverters, primarily inorganic phosphors, are critical components in white solid-state LED-b... more Downconverters, primarily inorganic phosphors, are critical components in white solid-state LED-based lighting and liquid crystal display backlights. Research efforts have led to a fundamental understanding of a downconverter's absorption, photoluminescence, and efficiency as a function of composition, structure, and processing conditions. However, considerably less work has focused on the reliability of phosphors once they are incorporated into LED packages. Solving these issues is often the final step before the commercialization of new materials, but the significant resources and time required to evaluate and mitigate materials failure are rarely discussed in the literature. In this Perspective, we discuss the need for conducting downconverter reliability testing and the potential of accelerating, screening, and understanding downconverter failure modes. Our focus highlights the mechanisms of failure and discusses how this influences materials selection and the design of different LED packages. We also stress the potential for accelerated reliability testing protocols and note the potential role first-principles calculations and data-driven models could play in establishing the compositional-processing trends for different aspects of downconverter reliability. We close with possible research directions that could improve downconverter reliability and emphasize the importance of assessing a material's (chemical) stability where multiple manufacturing and processing steps can dictate system performance.

Advanced Materials, Feb 23, 2010

Polymer solar cells are a promising alternative to future photovoltaic applications due to variou... more Polymer solar cells are a promising alternative to future photovoltaic applications due to various advantages such as low cost and ease of processing. [1-3] Ever since the invention of bulk heterojunctions (BHJs) [4-7] utilizing a blend of donor polymer and acceptor fullerene derivative, there have been increasing efforts to enhance the power-conversion efficiency (PCE) of polymer solar cells. However, narrow absorption range and low carrier mobility of polymer solar cells limit the thickness of the active layer and, hence, the absorption efficiency. One of the strategies to increase its efficiency is to make use of tandem architectures. [8-12] In a two-terminal tandem solar cell, two subcells with complementary absorption bands are connected in series through an interlayer that acts as a recombination zone for electrons from one subcell and holes from the other. The series connection leads to summation of the open-circuit voltage (V OC); however, the overall current is limited by the subcell that delivers the smaller photocurrent. [8,13] Therefore, the photocurrentmatching criterion between the two subcells must be satisfied for efficient working of a two-terminal tandem cell. Because of the current-matching criterion, the two subcells might not be readily incorporated into a tandem cell under their optimal conditions. An ideal tandem structure would consist of two subcells that operate separately, so that the efficiency of the tandem cell is simply the sum of the efficiencies of the two subcells. To circumvent the challenge of photocurrent matching in the two-terminal architecture, two solar cells can be stacked together to form a four-terminal tandem cell. [14,15] In this case, the two cells can be connected either in series, to add up the V OC , or connected in parallel (shown in Fig. 1a) to add up the short-circuit current (J SC). However, in these devices, considerable optical losses are encountered because of the presence of an additional indium tin oxide (ITO)-coated glass substrate and semitransparent metal electrode that compensates for the gain from absorption. In this manuscript, tandem structures with a three-terminal (3T) configuration are demonstrated, as seen in Figure 1b, in which two subcells are connected in parallel through a transparent conducting interlayer that acts as a common electrode to the two subcells. The advantage of this structure is that it is convenient to characterize the two subcells independently as well as when connected in parallel. If two subcells with similar V OC are used, they can be connected in parallel to sum the J SC without satisfying photocurrent match. Even if the V OC of the two subcells does not match, power can be drawn separately from the two cells. The 3T structure is relatively simple and straightforward. 3T tandem photovoltaic cells with either a common-anode or a COMMUNICATION www.MaterialsViews.com

ECS Journal of Solid State Science and Technology, Sep 7, 2017

In this report, we discuss the effect of synthesis on the concentration quenching of K 2 SiF 6 :M... more In this report, we discuss the effect of synthesis on the concentration quenching of K 2 SiF 6 :Mn 4+ phosphors. Comparing two different synthesis methods, we demonstrate that Mn concentration can be increased by a factor of three before the onset of concentration quenching. These effects are analyzed using literature models for concentration quenching that show that large reductions in defect concentrations are responsible for the improvements in concentration quenching along with a possible change in the mechanism behind concentration quenching. The practical implications for LED packages using high Mn 4+ concentration K 2 SiF 6 :Mn 4+ phosphor with high quantum efficiencies are also discussed along with suggestions about pitfalls in the analysis of concentration quenching.

Energy and Environmental Science, 2011

In this article, we review general approaches, challenges and solutions, as well as latest resear... more In this article, we review general approaches, challenges and solutions, as well as latest research progress, in the field of polymer tandem solar cells. Despite many limiting factors ranging from processing issues to characterization issues, it has been shown that high efficiency and in-depth understanding of tandem cell operation can be achieved via smart device design and analysis, and interface engineering. We further discuss in detail the operation principles and design rules of polymer tandem cells, revealing a bright future to reach double-digit power conversion efficiency from polymer tandem cells.

ECS Journal of Solid State Science and Technology, Nov 8, 2019

Macromolecules, Nov 9, 2009

Meeting abstracts, 2015

Mn4+ doped complex fluoride based red phosphors have the potential to improve efficiency and colo... more Mn4+ doped complex fluoride based red phosphors have the potential to improve efficiency and color quality (CRI & R9) in LED lighting systems and enhance color gamut for LED displays. Owing to low absorption of Mn4+, high Mn4+ doping concentration is necessary to enable low CCT applications. One of the issues with Mn4+ doped phosphors is their high temperature and high humidity (HTHH) stability which is even more important for high Mn4+ doping. Here we will discuss the various post synthesis treatment techniques that help improve HTHH stability of Mn4+ doped fluoride phosphors.

Meeting abstracts, 2016

Absorption and scattering coefficients are, together with quantum efficiency and spectral respons... more Absorption and scattering coefficients are, together with quantum efficiency and spectral response, fundamental parameters that describe fluorescent phosphors and their behavior in white LEDs, fluorescent lamps and displays. Simple reflectance measurements typically performed in industry often confound the absorption and scattering coefficients and understanding their individual contributions can be challenging. In this talk we will describe techniques that allow, in a simple and precise manner, to measure accurately and independently the absorption and scattering coefficients relative to a reference sample.

Meeting abstracts, 2015

Red phosphors based upon Mn4+-doped complex fluorides are currently being implemented in a variet... more Red phosphors based upon Mn4+-doped complex fluorides are currently being implemented in a variety of LED display and lighting applications. These phosphors can enable high CRI (for lighting) or high color gamut (for LCD backlit displays) without significant efficiency losses. However, the properties of these phosphors can significantly differ based upon the specific phosphor host. One example is in the temperature quenching of luminescence where T0.5, the temperature where emission intensity reaches 50% of the room temperature value. Different Mn4+-doped complex fluoride phosphors can have T0.5 values that vary by more than 100oC. In many applications, these differences in thermal quenching can prevent the use of these materials In this presentation, we discuss the various properties of numerous Mn4+-doped complex fluoride phosphors as a function of temperature. Correlations between composition, vibrational properties, and luminescence properties will be discussed and analyzed. Finally, the presence (or absence) of thermal quenching as a function of Mn4+ concentration will also be presented.

Journal of Applied Physics

Downconverters, primarily inorganic phosphors, are critical components in white solid-state LED-b... more Downconverters, primarily inorganic phosphors, are critical components in white solid-state LED-based lighting and liquid crystal display backlights. Research efforts have led to a fundamental understanding of a downconverter's absorption, photoluminescence, and efficiency as a function of composition, structure, and processing conditions. However, considerably less work has focused on the reliability of phosphors once they are incorporated into LED packages. Solving these issues is often the final step before the commercialization of new materials, but the significant resources and time required to evaluate and mitigate materials failure are rarely discussed in the literature. In this Perspective, we discuss the need for conducting downconverter reliability testing and the potential of accelerating, screening, and understanding downconverter failure modes. Our focus highlights the mechanisms of failure and discusses how this influences materials selection and the design of diff...

Applied Physics Letters, Nov 26, 2007

Proceedings of SPIE, Oct 8, 2014

Efficient light extraction from organic light emitting diode (OLED) is challenging and efforts ar... more Efficient light extraction from organic light emitting diode (OLED) is challenging and efforts are being made to come up with efficient & cost effective outcoupling techniques. We demonstrate 50% EQE entitlement from solution processed white OLEDs compared to 33% EQE observed in devices, implying that there is plenty of room to improve the efficiency of white OLEDs. We present challenges in efficient light extraction from solution processed OLEDs that need to be overcome to close the efficiency gap. We also demonstrate a novel characterization technique that is effective in estimating the light extraction efficiency of outcoupling films and can expedite the selection and optimization of various light extraction approaches without the need to build OLEDs.

Organic Electronics, Nov 1, 2015

One of the key challenges in organic light-emitting diodes (OLEDs) for lighting applications is e... more One of the key challenges in organic light-emitting diodes (OLEDs) for lighting applications is efficient light extraction from the planar, multi-layered OLED stack. Several different light extraction approaches are being explored currently by researchers, however characterizing light extraction films after fabricating OLEDs is not a viable approach when the outcoupling films have large surface roughness and is time consuming as well. Here we apply prism coupling method (PCM), a simple and elegant tool, to characterize outcoupling films. We show the effectiveness of PCM in estimating light extraction efficiency of outcoupling films. PCM can expedite selection and optimization of various light extraction approaches without the need to build OLEDs. The experimental results are corroborated by the optical simulations done using ray tracing method taking into account Mie scattering from wavelength sized spherical inclusions in an outcoupling film.

Applied Physics Letters, Mar 5, 2007

An efficient photovoltaic heterojunction of tetracene and fullerene has been investigated, and hi... more An efficient photovoltaic heterojunction of tetracene and fullerene has been investigated, and high performance organic solar cells have been demonstrated by thermal deposition and successive heat treatment. After the heat treatment, the open circuit voltage of the devices was enhanced greatly and at the same time the photocurrent remained almost unchanged. The series resistance of the devices was reduced and the fill factor was slightly enhanced. Consequently, the power conversion efficiency was improved from 1.7% to 2.2%. The preliminary conclusion for this enhancement is due to the part crystallization of the tetracene layer and consequent morphological change, which were supported by atomic force microscopy images, absorption spectra, and x-ray diffraction analysis. The part crystallization results in increase in hole mobility as evidenced by hole mobility measurements.

Advanced Materials, Feb 23, 2010

Polymer solar cells are a promising alternative to future photovoltaic applications due to variou... more Polymer solar cells are a promising alternative to future photovoltaic applications due to various advantages such as low cost and ease of processing. [1-3] Ever since the invention of bulk heterojunctions (BHJs) [4-7] utilizing a blend of donor polymer and acceptor fullerene derivative, there have been increasing efforts to enhance the power-conversion efficiency (PCE) of polymer solar cells. However, narrow absorption range and low carrier mobility of polymer solar cells limit the thickness of the active layer and, hence, the absorption efficiency. One of the strategies to increase its efficiency is to make use of tandem architectures. [8-12] In a two-terminal tandem solar cell, two subcells with complementary absorption bands are connected in series through an interlayer that acts as a recombination zone for electrons from one subcell and holes from the other. The series connection leads to summation of the open-circuit voltage (V OC); however, the overall current is limited by the subcell that delivers the smaller photocurrent. [8,13] Therefore, the photocurrentmatching criterion between the two subcells must be satisfied for efficient working of a two-terminal tandem cell. Because of the current-matching criterion, the two subcells might not be readily incorporated into a tandem cell under their optimal conditions. An ideal tandem structure would consist of two subcells that operate separately, so that the efficiency of the tandem cell is simply the sum of the efficiencies of the two subcells. To circumvent the challenge of photocurrent matching in the two-terminal architecture, two solar cells can be stacked together to form a four-terminal tandem cell. [14,15] In this case, the two cells can be connected either in series, to add up the V OC , or connected in parallel (shown in Fig. 1a) to add up the short-circuit current (J SC). However, in these devices, considerable optical losses are encountered because of the presence of an additional indium tin oxide (ITO)-coated glass substrate and semitransparent metal electrode that compensates for the gain from absorption. In this manuscript, tandem structures with a three-terminal (3T) configuration are demonstrated, as seen in Figure 1b, in which two subcells are connected in parallel through a transparent conducting interlayer that acts as a common electrode to the two subcells. The advantage of this structure is that it is convenient to characterize the two subcells independently as well as when connected in parallel. If two subcells with similar V OC are used, they can be connected in parallel to sum the J SC without satisfying photocurrent match. Even if the V OC of the two subcells does not match, power can be drawn separately from the two cells. The 3T structure is relatively simple and straightforward. 3T tandem photovoltaic cells with either a common-anode or a COMMUNICATION www.MaterialsViews.com

In this article, we review general approaches, challenges and solutio progress, in the field of p... more In this article, we review general approaches, challenges and solutio progress, in the field of polymer tandem solar cells. Despite many lim processing issues to characterization issues, it has been shown that h understanding of tandem cell operation can be achieved via smart d interface engineering. We further discuss in detail the operation princ tandem cells, revealing a bright future to reach double-digit power co tandem cells. 1.

Sid's Digest Of Technical Papers, Jun 1, 2015

The red‐line emission of PFS, K2SiF6:Mn4+, phosphor centered at 631 nm results in phosphor conver... more The red‐line emission of PFS, K2SiF6:Mn4+, phosphor centered at 631 nm results in phosphor converted LED package efficacy improvements of >10% relative to packages using conventional Eu2% doped nitride broad‐band red‐emitting phosphors. Improvements in absorption, quantum efficiency and stability under high humidity and high light flux will be presented that have resulted in commercialization of this material under GE TriGain™ Technology.

Journal of Applied Physics, Nov 21, 2022

Downconverters, primarily inorganic phosphors, are critical components in white solid-state LED-b... more Downconverters, primarily inorganic phosphors, are critical components in white solid-state LED-based lighting and liquid crystal display backlights. Research efforts have led to a fundamental understanding of a downconverter's absorption, photoluminescence, and efficiency as a function of composition, structure, and processing conditions. However, considerably less work has focused on the reliability of phosphors once they are incorporated into LED packages. Solving these issues is often the final step before the commercialization of new materials, but the significant resources and time required to evaluate and mitigate materials failure are rarely discussed in the literature. In this Perspective, we discuss the need for conducting downconverter reliability testing and the potential of accelerating, screening, and understanding downconverter failure modes. Our focus highlights the mechanisms of failure and discusses how this influences materials selection and the design of different LED packages. We also stress the potential for accelerated reliability testing protocols and note the potential role first-principles calculations and data-driven models could play in establishing the compositional-processing trends for different aspects of downconverter reliability. We close with possible research directions that could improve downconverter reliability and emphasize the importance of assessing a material's (chemical) stability where multiple manufacturing and processing steps can dictate system performance.

Advanced Materials, Feb 23, 2010

Polymer solar cells are a promising alternative to future photovoltaic applications due to variou... more Polymer solar cells are a promising alternative to future photovoltaic applications due to various advantages such as low cost and ease of processing. [1-3] Ever since the invention of bulk heterojunctions (BHJs) [4-7] utilizing a blend of donor polymer and acceptor fullerene derivative, there have been increasing efforts to enhance the power-conversion efficiency (PCE) of polymer solar cells. However, narrow absorption range and low carrier mobility of polymer solar cells limit the thickness of the active layer and, hence, the absorption efficiency. One of the strategies to increase its efficiency is to make use of tandem architectures. [8-12] In a two-terminal tandem solar cell, two subcells with complementary absorption bands are connected in series through an interlayer that acts as a recombination zone for electrons from one subcell and holes from the other. The series connection leads to summation of the open-circuit voltage (V OC); however, the overall current is limited by the subcell that delivers the smaller photocurrent. [8,13] Therefore, the photocurrentmatching criterion between the two subcells must be satisfied for efficient working of a two-terminal tandem cell. Because of the current-matching criterion, the two subcells might not be readily incorporated into a tandem cell under their optimal conditions. An ideal tandem structure would consist of two subcells that operate separately, so that the efficiency of the tandem cell is simply the sum of the efficiencies of the two subcells. To circumvent the challenge of photocurrent matching in the two-terminal architecture, two solar cells can be stacked together to form a four-terminal tandem cell. [14,15] In this case, the two cells can be connected either in series, to add up the V OC , or connected in parallel (shown in Fig. 1a) to add up the short-circuit current (J SC). However, in these devices, considerable optical losses are encountered because of the presence of an additional indium tin oxide (ITO)-coated glass substrate and semitransparent metal electrode that compensates for the gain from absorption. In this manuscript, tandem structures with a three-terminal (3T) configuration are demonstrated, as seen in Figure 1b, in which two subcells are connected in parallel through a transparent conducting interlayer that acts as a common electrode to the two subcells. The advantage of this structure is that it is convenient to characterize the two subcells independently as well as when connected in parallel. If two subcells with similar V OC are used, they can be connected in parallel to sum the J SC without satisfying photocurrent match. Even if the V OC of the two subcells does not match, power can be drawn separately from the two cells. The 3T structure is relatively simple and straightforward. 3T tandem photovoltaic cells with either a common-anode or a COMMUNICATION www.MaterialsViews.com

ECS Journal of Solid State Science and Technology, Sep 7, 2017

In this report, we discuss the effect of synthesis on the concentration quenching of K 2 SiF 6 :M... more In this report, we discuss the effect of synthesis on the concentration quenching of K 2 SiF 6 :Mn 4+ phosphors. Comparing two different synthesis methods, we demonstrate that Mn concentration can be increased by a factor of three before the onset of concentration quenching. These effects are analyzed using literature models for concentration quenching that show that large reductions in defect concentrations are responsible for the improvements in concentration quenching along with a possible change in the mechanism behind concentration quenching. The practical implications for LED packages using high Mn 4+ concentration K 2 SiF 6 :Mn 4+ phosphor with high quantum efficiencies are also discussed along with suggestions about pitfalls in the analysis of concentration quenching.

Energy and Environmental Science, 2011

In this article, we review general approaches, challenges and solutions, as well as latest resear... more In this article, we review general approaches, challenges and solutions, as well as latest research progress, in the field of polymer tandem solar cells. Despite many limiting factors ranging from processing issues to characterization issues, it has been shown that high efficiency and in-depth understanding of tandem cell operation can be achieved via smart device design and analysis, and interface engineering. We further discuss in detail the operation principles and design rules of polymer tandem cells, revealing a bright future to reach double-digit power conversion efficiency from polymer tandem cells.

ECS Journal of Solid State Science and Technology, Nov 8, 2019

Macromolecules, Nov 9, 2009

Meeting abstracts, 2015

Mn4+ doped complex fluoride based red phosphors have the potential to improve efficiency and colo... more Mn4+ doped complex fluoride based red phosphors have the potential to improve efficiency and color quality (CRI & R9) in LED lighting systems and enhance color gamut for LED displays. Owing to low absorption of Mn4+, high Mn4+ doping concentration is necessary to enable low CCT applications. One of the issues with Mn4+ doped phosphors is their high temperature and high humidity (HTHH) stability which is even more important for high Mn4+ doping. Here we will discuss the various post synthesis treatment techniques that help improve HTHH stability of Mn4+ doped fluoride phosphors.

Meeting abstracts, 2016

Absorption and scattering coefficients are, together with quantum efficiency and spectral respons... more Absorption and scattering coefficients are, together with quantum efficiency and spectral response, fundamental parameters that describe fluorescent phosphors and their behavior in white LEDs, fluorescent lamps and displays. Simple reflectance measurements typically performed in industry often confound the absorption and scattering coefficients and understanding their individual contributions can be challenging. In this talk we will describe techniques that allow, in a simple and precise manner, to measure accurately and independently the absorption and scattering coefficients relative to a reference sample.

Meeting abstracts, 2015

Red phosphors based upon Mn4+-doped complex fluorides are currently being implemented in a variet... more Red phosphors based upon Mn4+-doped complex fluorides are currently being implemented in a variety of LED display and lighting applications. These phosphors can enable high CRI (for lighting) or high color gamut (for LCD backlit displays) without significant efficiency losses. However, the properties of these phosphors can significantly differ based upon the specific phosphor host. One example is in the temperature quenching of luminescence where T0.5, the temperature where emission intensity reaches 50% of the room temperature value. Different Mn4+-doped complex fluoride phosphors can have T0.5 values that vary by more than 100oC. In many applications, these differences in thermal quenching can prevent the use of these materials In this presentation, we discuss the various properties of numerous Mn4+-doped complex fluoride phosphors as a function of temperature. Correlations between composition, vibrational properties, and luminescence properties will be discussed and analyzed. Finally, the presence (or absence) of thermal quenching as a function of Mn4+ concentration will also be presented.

Journal of Applied Physics

Downconverters, primarily inorganic phosphors, are critical components in white solid-state LED-b... more Downconverters, primarily inorganic phosphors, are critical components in white solid-state LED-based lighting and liquid crystal display backlights. Research efforts have led to a fundamental understanding of a downconverter's absorption, photoluminescence, and efficiency as a function of composition, structure, and processing conditions. However, considerably less work has focused on the reliability of phosphors once they are incorporated into LED packages. Solving these issues is often the final step before the commercialization of new materials, but the significant resources and time required to evaluate and mitigate materials failure are rarely discussed in the literature. In this Perspective, we discuss the need for conducting downconverter reliability testing and the potential of accelerating, screening, and understanding downconverter failure modes. Our focus highlights the mechanisms of failure and discusses how this influences materials selection and the design of diff...

Applied Physics Letters, Nov 26, 2007

Proceedings of SPIE, Oct 8, 2014

Efficient light extraction from organic light emitting diode (OLED) is challenging and efforts ar... more Efficient light extraction from organic light emitting diode (OLED) is challenging and efforts are being made to come up with efficient & cost effective outcoupling techniques. We demonstrate 50% EQE entitlement from solution processed white OLEDs compared to 33% EQE observed in devices, implying that there is plenty of room to improve the efficiency of white OLEDs. We present challenges in efficient light extraction from solution processed OLEDs that need to be overcome to close the efficiency gap. We also demonstrate a novel characterization technique that is effective in estimating the light extraction efficiency of outcoupling films and can expedite the selection and optimization of various light extraction approaches without the need to build OLEDs.

Organic Electronics, Nov 1, 2015

One of the key challenges in organic light-emitting diodes (OLEDs) for lighting applications is e... more One of the key challenges in organic light-emitting diodes (OLEDs) for lighting applications is efficient light extraction from the planar, multi-layered OLED stack. Several different light extraction approaches are being explored currently by researchers, however characterizing light extraction films after fabricating OLEDs is not a viable approach when the outcoupling films have large surface roughness and is time consuming as well. Here we apply prism coupling method (PCM), a simple and elegant tool, to characterize outcoupling films. We show the effectiveness of PCM in estimating light extraction efficiency of outcoupling films. PCM can expedite selection and optimization of various light extraction approaches without the need to build OLEDs. The experimental results are corroborated by the optical simulations done using ray tracing method taking into account Mie scattering from wavelength sized spherical inclusions in an outcoupling film.

Applied Physics Letters, Mar 5, 2007

An efficient photovoltaic heterojunction of tetracene and fullerene has been investigated, and hi... more An efficient photovoltaic heterojunction of tetracene and fullerene has been investigated, and high performance organic solar cells have been demonstrated by thermal deposition and successive heat treatment. After the heat treatment, the open circuit voltage of the devices was enhanced greatly and at the same time the photocurrent remained almost unchanged. The series resistance of the devices was reduced and the fill factor was slightly enhanced. Consequently, the power conversion efficiency was improved from 1.7% to 2.2%. The preliminary conclusion for this enhancement is due to the part crystallization of the tetracene layer and consequent morphological change, which were supported by atomic force microscopy images, absorption spectra, and x-ray diffraction analysis. The part crystallization results in increase in hole mobility as evidenced by hole mobility measurements.

Advanced Materials, Feb 23, 2010

Polymer solar cells are a promising alternative to future photovoltaic applications due to variou... more Polymer solar cells are a promising alternative to future photovoltaic applications due to various advantages such as low cost and ease of processing. [1-3] Ever since the invention of bulk heterojunctions (BHJs) [4-7] utilizing a blend of donor polymer and acceptor fullerene derivative, there have been increasing efforts to enhance the power-conversion efficiency (PCE) of polymer solar cells. However, narrow absorption range and low carrier mobility of polymer solar cells limit the thickness of the active layer and, hence, the absorption efficiency. One of the strategies to increase its efficiency is to make use of tandem architectures. [8-12] In a two-terminal tandem solar cell, two subcells with complementary absorption bands are connected in series through an interlayer that acts as a recombination zone for electrons from one subcell and holes from the other. The series connection leads to summation of the open-circuit voltage (V OC); however, the overall current is limited by the subcell that delivers the smaller photocurrent. [8,13] Therefore, the photocurrentmatching criterion between the two subcells must be satisfied for efficient working of a two-terminal tandem cell. Because of the current-matching criterion, the two subcells might not be readily incorporated into a tandem cell under their optimal conditions. An ideal tandem structure would consist of two subcells that operate separately, so that the efficiency of the tandem cell is simply the sum of the efficiencies of the two subcells. To circumvent the challenge of photocurrent matching in the two-terminal architecture, two solar cells can be stacked together to form a four-terminal tandem cell. [14,15] In this case, the two cells can be connected either in series, to add up the V OC , or connected in parallel (shown in Fig. 1a) to add up the short-circuit current (J SC). However, in these devices, considerable optical losses are encountered because of the presence of an additional indium tin oxide (ITO)-coated glass substrate and semitransparent metal electrode that compensates for the gain from absorption. In this manuscript, tandem structures with a three-terminal (3T) configuration are demonstrated, as seen in Figure 1b, in which two subcells are connected in parallel through a transparent conducting interlayer that acts as a common electrode to the two subcells. The advantage of this structure is that it is convenient to characterize the two subcells independently as well as when connected in parallel. If two subcells with similar V OC are used, they can be connected in parallel to sum the J SC without satisfying photocurrent match. Even if the V OC of the two subcells does not match, power can be drawn separately from the two cells. The 3T structure is relatively simple and straightforward. 3T tandem photovoltaic cells with either a common-anode or a COMMUNICATION www.MaterialsViews.com

In this article, we review general approaches, challenges and solutio progress, in the field of p... more In this article, we review general approaches, challenges and solutio progress, in the field of polymer tandem solar cells. Despite many lim processing issues to characterization issues, it has been shown that h understanding of tandem cell operation can be achieved via smart d interface engineering. We further discuss in detail the operation princ tandem cells, revealing a bright future to reach double-digit power co tandem cells. 1.