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We propose a series-connected hybrid tandem solar cell which consists of an organic solar cell (P3HT/PCBM) as the top cell and an organic/crystalline silicon hybrid solar cell (PEDOT:PSS/c-Si nanowires) as the bottom cell. Based on the device structure, the organic materials can be directly spun-cast onto the inorganic silicon substrate with thermally evaporated metal contacts, making solution-based processes possible for rapid and low-cost production. With a proper design, the hybrid device architecture can achieve a high open-circuit voltage and junction-matched photocurrent, offering a promising approach for next-generation high-efficiency photovoltaics. In this work, we established a device model to investigate the photovoltaic characteristics of the proposed hybrid tandem solar cells by combining the organic and hybrid silicon solar cells with a hypothetic recombination layer (RL). First, the model of single junction solar cells is fitted to the current-voltage curve of fabricated devices. Next, we investigate the properties of the RL between the sub-cells and observe strong correlations with the photovoltaic performance of tandem cells. In our preliminary model, we have realized a cell with an open-circuit voltage (Voc), short-circuit current (Jsc), fill-factor (FF) and power conversion efficiency (PCE) of 1.093 V, 9.715 mA/cm2, 43.725 % and 4.644 %, respectively. We will further tailor the properties of the RL, the active-layer thickness of sub-cells, as well as the band alignment, in order to achieve practical device designs. Currently, the characteristics of real hybrid tandem solar cells remain significantly lower than the simulation result. The reason of such limited cell performance is the poor interfacial contact, which makes it difficult to provide efficient recombination and transport for electrons and holes generated from sub-cells. A number of challenging issues, including interface physics and device design will be discussed.

Metallic nanostructures provide new possibilities for the photoelectric conversion enhancement of solar cells due to the existence of localized surface plasmon. Since the carrier generation rate is proportional to the electric intensity, a resonant plasmonic excitation can induce a strong local field around metallic nanostructures, hence increasing the optical absorption of the surrounding semiconductor. Previously, the incorporation of metallic nanostructures on the rear side of thin film solar cells has been demonstrated with photocurrent spectrum mapped to the photonic band structure. However, the device structure and the far-field measurement techniques cannot distinguish the photovoltaic enhancement from either or both the increased diffraction or localized surface plasmon resonance. In this work, we perform a three-dimensional finite element simulation to monitor enhanced optical absorption induced by localized surface plasmon and optical diffraction of periodic gold nanospheres incorporated onto the front surface of a silicon solar cell. The calculated structures include gold nanospheres with various diameters and densities. First, the resonance peak of localized surface plasmon is tailored to match the absorption spectrum of silicon. Next, the electric field distribution is calculated explicitly, where we observe enhanced optical absorption by more than 2 fold within a local volume underneath a gold nanosphere with a diameter of 100 nm, compared to the reference bare silicon. The enhancement is induced by the localized surface plasmon. However, the total absorption of the bulk silicon with gold particles is reduced at the resonance due to a phase cancellation resulting from the transmitted field and the diffracted field. It is therefore important to design the dimensions of nanospheres to achieve broadband absorption enhancement. Finally, the density of nanospheres is also optimized and achieves a maximal photocurrent conversion of 18.6 mA/cm2 at 3.2×1019 cm-2, which represents a compromise between forward scattering and metallic shadowing.

Hybrid Graphene/Si Schottky junction solar cells are inexpensive alternatives for photovoltaics due to easy and rapid fabrication processes. However, one of the main challenges for graphene-based Schottky junction solar cells is the large sheet resistance of single-layer graphene, leading to low fill factor. Here, we demonstrate multi-layer graphene-based Schottky junction solar cell, which are prepared by stacking monolayer graphene via a chemical vapor deposition (CVD) method on copper foils. The objectives are to improve charge transport and decrease the series resistance of the hybrid solar cell. We present the Raman spectrum results of monolayer graphene and stacked multilayer graphene films, and observe no bonding interaction between the stacked layers. For the optical and electrical properties, the sheet resistance of stacked multilayer graphene is reduced from 3000Ω/sq to 1500Ω/sq and 750Ω/sq with two and three layers, respectively. The reduction of the sheet resistance approaches the properties of highly-oriented pyrolytic graphite (HOPG) when the layer number is more than five. Moreover, from the optical transmittance measurement, the absorption of a monolayer graphene is about 3% at the 555 nm wavelength, giving rise to a transmittance of 90% for triple-layer graphene. The hybrid silicon solar cell with a single-layer graphene achieves a power conversion efficiency of 0.9% with a short-circuit current of 8.75 mA/cm2, open-circuit voltage of 0.25V, and fill factor of 39%. The external quantum efficiency measurement reveals an integrated photocurrent of 18.3 mA/cm2, which further confirms the potential of the hybrid photovoltaic approach. Device fabrication with multi-layer graphene is still in process, and more data will be presented. Other characterization techniques such as capacitance-voltage and capacitance-frequency measurements will be taken to analyze the performance of the single- and multilayer graphene solar cells.

Hybrid organic-inorganic heterojunction solar cells based on silicon nanowires (SiNWs) are promising candidates for next-generation photovoltaics owing to potentials for low fabrication cost and high efficiency. The SiNW array, fabricated by a simple metal-assisted wet chemical etching method, produces a large surface-area-to-volume ratio, hence allowing efficient light harvesting and charge collection via the formation of a core-sheath p-n junction. However, previously reported power conversion efficiencies (PCEs) are approximately capped at 10%, which is largely depicted by the interface defect densities that limit the open-circuit voltage (Voc) and fill factor (FF). In this work, we introduce a solution-processed, intermediate 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) layer to mitigate the interface recombination loss for hybrid heterojunction solar cells consisted of SiNWs and conjugate polymer poly(3,4-ethylenedioxy-thiophene): poly(styrenesulfonate) (PEDOT:PSS). A record PCE of 11.0% is achieved in contrast to 9.6% from a reference counterpart without TAPC, which represents an enhancement factor of 14.2% ascribed to noticeable improvement in the Voc and FF. The result is further supported by examining indicators for the interface quality via a suppressed dark saturation current and an enhanced minority carrier lifetime which exhibits an increase from 84 μsec without TAPC to 87 μsec with TAPC.

Hybrid solar cells combining organic polymers and inorganic semiconductors are extensively investigated recently due to relatively inexpensive cost and simple fabrication processes. In this work, we demonstrate organic/inorganic hybrid heterojunction solar cells based on gallium arsenide (GaAs) substrate and conjugated polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS). First we performed a one-dimensional device simulation based on a self-consistent Poisson and drift-diffusion solver to survey the band alignment between the conductive polymer and GaAs materials and achieve a practical device design. Second, for device fabrication, we prepare a cleaned one-side-polished planar GaAs wafer, followed by thermal evaporation of back-side metal using either Aluminum or titanium/gold as the cathode. Next, PEDOT:PSS is spun-cast onto the wafer and annealed at 115°C for 10 minutes. To improve carrier conduction, we use a self-assembled polystyrene (PS) nanosphere lithography technique to form the sacrificial mask layer, and perform anisotropic metal-assisted chemical etching on GaAs substrates. Various nanostructures such as nanowires or nanorods allow the conformal p-n heterojunction formation at the interface of organic/inorganic semiconductors, which can be beneficial for both light absorption and carrier collection. The optical and electrical characteristics such as reflectance, current voltage, and external quantum efficiency are measured. Currently, we achieve a 3.2% power conversion efficiency with an open-circuit voltage of 0.565 V, short-circuit current of 8.95 mA/cm2, and a fill factor of 63.29% under a simulated AM1.5G illumination for planar substrates. Device fabrication with GaAs nanowires is still in process and more data will be presented.

Silicon-based hybrid solar cells have garnered extensive attentions in the photovoltaics industry due to easy processing attributes and high optical absorption and outstanding carrier mobility of silicon. Among all, indium tin oxide (ITO)/silicon solar cells have achieved a power conversion efficiency of 13% due to excellent conductivity, transmittance and applicable surface potential of ITO. However, the cost of ITO has risen significantly recently due to the deficiency of indium. Therefore, graphene has been an inexpensive alternative to ITO. For solar cell applications, graphene plays an important role as transparent electrodes (TE) with tunable work functions for efficient carrier collection. Therefore, graphene-based Schottky junction solar cells (SJSC) on crystalline silicon thin films hold great promises for low-cost photovoltaics owing to potentials for high efficiency and rapid production on flexible substrates. According to previous reports, the key factors to achieve a highly efficient SJSC include excellent transparence and conductance, as well as tunable work functions. Herein, we demonstrate a single layer graphene/n-Si Schottky junction solar cell that exhibits a power conversion efficiency (PCE) of 1.2 % under one-sun AM 1.5G illumination, and an integrated short-circuit photocurrent of 18.3 mA/cm2 from the external quantum efficiency measurement. The transmittance of the monolayer graphene in this device is over 97 % and the sheet resistance is around 800 to 1200 Ω/□. Furthermore, we investigate a doping method involving bis(trifluoromethanesulfonyl)-amid (TFSA) for the monolayer graphene to improve the separation and collection of photogenerated carriers in the SJSC. The preliminary data show that the sheet resistance is decreased rapidly from 1200 to 300 Ω/□. and the surface potential is also adjusted by the chemical doping. Currently, device fabrication with doped monolayer graphene is still in process and complete characterization data will be presented.

Mono- and multi-crystalline silicon photovoltaics currently still hold more than 80% market share because of the non-toxic, abundant material resources used, and their long-term stabilities. However, the cost of solar power is still more than three times that of fossil fuels, which necessitates a further reduction to accelerate its widespread use. It has been estimated that cell fabrication consumes 30% of the total manufacturing cost due to energy intensive semiconductor processes, such as high temperature furnace for doping, electrodes co-firing, high-vacuum chemical deposition, etc. Therefore, the organic-inorganic hybrid cell concept has been proposed to take advantage of the solution-based processes for rapid and low-cost production and the wide absorption spectrum of silicon. In this work, we demonstrate a hybrid heterojunction solar cell based on the structure of conductive polymer PEDOT:PSS spun cast on n-type crystalline silicon nanorod (SiNR) arrays with periodic arrangements. The nanorod arrays are fabricated by electron beam (E-beam) lithography followed by reactive-ion etching (RIE), which show capability to enhance light harvesting. In addition, SiNRs and PEDOT:PSS can form core-shell structure that provides a large p-n junction area for carrier separation and collection. We measured the optical and photovoltaic characteristics of these devices under a simulated class A solar simulator with a calibrated illumination intensity of 1000 W/m2 for the AM1.5G solar spectrum. A post-RIE damage removal etching (DRE) is subsequently introduced in order to mitigate the surface recombination issues and also alter the surface reflection due to modifications in the nanorod side-wall profile. Finally, we show that the DRE treatment can effectively recover the carrier lifetime and dark current-voltage characteristics of SiNRs hybrid solar cells to resemble the planar counterpart without RIE damages.

Because of the global warming effect and energy crisis, the usage of solar energy gets more and more important in recent years. In particular, GaAs-based solar cells have been regarded as a promising candidate to provide high power conversion efficiency because of their direct band gap and strong absorption over the entire visible part of the solar spectrum. In this work, we demonstrate a hybrid design of traditional GaAs-based solar cell combined with colloidal quantum dots. Several photovoltaic parameters were enhanced, including the short circuit current density, fill factor, and power conversion efficiency, which were measured under white light illumination similar to the solar spectrum. With anti-reflective feature at long wavelength and down-conversion at high energy photons, the quantum dot effectively enhance the overall power conversion efficiency by as high as 25% compared to traditional GaAs-based device. The evolution of short-circuit current density and weighted reflectance as a function of the dilution factor of three type QDs has been investigated. The dilution factors of quantum dots solution also have been optimized for single junction GaAs devices. We also use the EQE enhancement spectrum to further distinguish between the photon down-conversion and antireflection capability of QDs. Finally, we believe this technology shall be a great candidate for next generation of highly efficient photovoltaic devices.