Organic and quantum dots (QDs) semiconductors are promising to build low-cost hybrid tandem solar cells since they are both fully solution-processable, and have tunable bandgaps and absorption spectra. The challenges for high-performance organic-QDs tandem solar cells are to balance the photocurrent in subcells and construct an efficient charge-recombination layer (CRL) to maximize the efficiency of the whole tandem cell. In this work, we report a top illuminated organic-QDs hybrid tandem solar cell that employs an organic-based front subcell and a PbS QDs-based back subcell where the organic absorber complements the absorption deficiency of QDs film in the range of 650–900 nm. The hybrid tandem solar cell is monolithically integrated and electrically connected with a Spiro-MeOTAD/MoO3/Ag/PEIE CRL. A conversion efficiency of 7.4% is achieved for the hybrid tandem cells. The tandem solar cells exhibit an open-circuit voltage of 1.12 V, which is nearly the sum of the VOC of individual subcells, and a fill factor up to 56%, confirming the effectiveness of CRL for building organic-QD hybrid tandem cells.Keywords: charge recombination layer; complementary absorption; hybrid tandem solar cells; top-illuminated structure;

Both single-junction and tandem organic photovoltaic cells have been well developed. A tandem cell contains two junctions with a charge recombination layer (CRL) inserted between the two junctions. So far, there is no detailed report on how the device will perform that contains two junctions but without a CRL in between. In this work, we report the photocurrent spectra and photovoltage output of the devices that contains two bulk-heterojunctions (BHJ) stacked directly on top of each other without a CRL. The top active layer is prepared by transfer printing. The photocurrent response spectra and photovoltage are found to be sensitive to stacking sequence and the selection of electron acceptors. The open-circuit voltage of the devices (up to 1.09 V) can be higher than the devices containing either junction layer. The new phenomenon in the new device architecture increases the versatility of the optoelectronic devices based on organic semiconductors.Keywords: layer stacking sequence; organic optoelectronic devices; photocurrent response spectra; photovoltage; transfer printing;

Fabrication of efficient colorful flexible polymer tandem solar cells remains a challenge. In this work, we report the use of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the top transparent electrode in tandem solar cells that in the meantime acts as an optical coating for light engineering. Modification of the thickness of the transfer-printed PEDOT:PSS allows tailoring the reflectance spectra and yields colorful polymer solar cells with easy processability. Compared to the strategies of adopting optical microcavities and photonic crystals, the use of PEDOT:PSS also offers the important advantage of excellent mechanical flexibility that enables the fabrication of flexible colorful solar cells. The fabricated colorful flexible polymer tandem solar cells with polymer electrodes display power conversion efficiency (PCE) values from 7.23% to 8.34% depending on the yielded color of the cells, which are among the highest values for reported colorful polymer solar cells.

The fabrication of thin layers of organic photoactive materials (typically ca. 100–200 nm thick) over large area is needed for the commercial realization of organic solar cells. This is challenging because defects on these thin layers can cause high leakage currents which lead to poor device performance and, ultimately, to poor device yield. Here, we report that organic solar cells with a tandem structure can display an increased tolerance to defects and are found less susceptible to parasitic area scaling-up effects compared to single-junction solar cells. We demonstrate 10.5 cm2 flexible tandem solar cells with a power conversion efficiency of 6.5% with a fabrication yield of over 90% in a laboratory environment. The high fabrication yield and good performance displayed by tandem organic solar cells suggest that despite their increased complexity, they could provide a viable path towards the commercial realization of efficient large-area organic solar cells.

The reproducibility of high-performance perovskite solar cells (PVSCs) remains a major obstacle. Herein, for the first time, we report the use of 2,6-dimethoxypyridine (2,6-Py) for interface chemistry engineering to fabricate reproducible high-efficiency planar perovskite solar cells. The 2,6-Py serves dual functions: (1) as a Lewis base enabling surface passivation of Lewis acid traps (e.g., under-coordinated Pb ions) without corroding the perovskite; (2) as a chemical dopant for [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) to improve its conductivity and mobility for efficient electron extraction and transport. Thus, through both the surface passivation of the perovskite layer and the doping of the electron transport layer with 2,6-Py, the resultant MAPbI3-based planar solar cells outperform the untreated devices with power conversion efficiency (PCE) significantly improved from 15.53% to 19.41%. The devices with the dual-function treatment yield effectively improved reproducibility with a narrow PCE distribution – that is, around 90% of the devices afford a PCE of over 17.50% (about 90% of the champion PCE), and also display enhanced air stability – that is, they maintain nearly 80% of their initial PCEs after 200 h in ambient air without any encapsulation.

AbstractThis study reports an effective amidine-type n-dopant of 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) that can universally dope electron acceptors, including PC61BM, N2200, and ITIC, by mixing the dopant with the acceptors in organic solvents or exposing the acceptor films in the dopant vapor. The doping mechanism is due to its strong electron-donating property that is also confirmed via the chemical reduction of PEDOT:PSS (yielding color change). The DBU doping considerably increases the electrical conductivity and shifts the Fermi levels up of the PC61BM films. When the DBU-doped PC61BM is used as an electron-transporting layer in perovskite solar cells, the n-doping removes the “S-shape” of J–V characteristics, which leads to the fill factor enhancement from 0.54 to 0.76. Furthermore, the DBU doping can effectively lower the threshold voltage and enhance the electron mobility of PC61BM-based n-channel field-effect transistors. These results show that the DBU can be a promising n-dopant for solution-processed electronics.

Organometal halide perovskites have shown excellent optoelectronic properties and have been used to demonstrate a variety of semiconductor devices. Colorful solar cells are desirable for photovoltaic integration in buildings and other aesthetically appealing applications. However, the realization of colorful perovskite solar cells is challenging because of their broad and large absorption coefficient that commonly leads to cells with dark-brown colors. Herein, for the first time, we report a simple and efficient strategy to achieve colorful perovskite solar cells by using the transparent conducting polymer (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS) as a top electrode and simultaneously as an spectrally selective antireflection coating. Vivid colors across the visible spectrum are attained by engineering optical interference effects among the transparent PEDOT:PSS polymer electrode, the hole-transporting layer and the perovskite layer. The colored perovskite solar cells display power conversion efficiency values from 12.8 to 15.1% (from red to blue) when illuminated from the FTO glass side and from 11.6 to 13.8% (from red to blue) when illuminated from the PEDOT:PSS side. The new approach provides an advanced solution for fabricating colorful perovskite solar cells with easy processing and high efficiency.Keywords: antireflection coating; building integrated photovoltaics; colorful; conducting polymer; Perovskite solar cells;

A low-cost, light-weight and flexible power supply is highly desirable for portable electronic devices. All-plastic solar cells in which all the layers are fabricated sequentially from organic synthetic inks could meet these requirements. Here, we report that fully solution-processed all-plastic multijunction solar cells can be easily fabricated layer by layer without the need for sophisticated patterning. The key for the high-yield fully solution-processed multijunction cells is the control and tuning of the conductivity of the charge-recombination layer of PEDOT:PSS/PEI. The all-plastic multijunction solar cells achieve a PCE of 6.1 ± 0.4% and a high open-circuit voltage of 5.37 V. These all-plastic multijunction solar cells are successfully used to drive liquid-crystal displays, full-color light-emitting diodes and for water splitting under different light illumination conditions.

Low dark current is critical to realize high-performance near-infrared organic photodetectors (NIR-OPDs). In general, organic photodetectors (OPDs) are with vacuum-deposited metals as the top electrode. The deposition of such metal would inevitably form doping to the organic active layer and thus yield high dark current. Herein, we employ transfer-printed conducting polymer (tp-CP) as the top electrode instead of the vacuum-deposited metal electrode. The photodetector with tp-CP electrode exhibits over two orders of magnitude lower dark current density than the device with the vacuum-deposited metal electrode. The photodetector with tp-CP electrode displays a responsivity of 0.37 A W−1 at 850 nm and a low dark current density of 3.0 nA cm−2 at −0.2 V based on a near-infrared (NIR) active layer of PMDPP3T:PC61BM that absorbs photons up to 1000 nm. The detectivity of the NIR photodetector reaches as high as over 1013 Jones. Furthermore, the NIR photodetector is double-side responsive to incident light, either from the bottom or the top electrode, because the top tp-CP electrode shows similar transparency as the bottom indium-tin oxide electrode.

Perovskite solar cells have been attracting a lot of attention because of their high power conversion efficiency and low-cost processing. However, device reproducibility has been a problem. The fabrication atmosphere is regarded as one of the possible reasons. So far, there has been a lack of direct evidence to prove which kind of atmosphere and how the atmosphere affects the device performance. Here, we report that the methylamine (MA, boiling point: −6 °C) that is used to synthesize the methylammonium iodide (MAI) could chemically reduce the PEDOT:PSS hole-transporting layer. After the reduction, a strong absorbance band appears at 400–1100 nm and the conductivity and work function simultaneously decrease. Furthermore, the MA-reduced PEDOT:PSS films are also found to be easily oxidized in air. The reduced work function of the PEDOT:PSS layer leads to poor hole collection and yields low open-circuit voltage, short-circuit current and power conversion efficiency of the perovskite solar cells. Therefore, though the MA vapor-containing fabrication atmosphere is beneficial to the performance of TiO2-based perovskite solar cells in which the bottom electrode collects the electrons, it is detrimental to that of PEDOT:PSS-based solar cells.

Building a tandem structure is an effective strategy to enhance the photovoltaic performance of solar cells. In the realization of a two-terminal tandem device, the charge recombination layer (CRL) plays an essential role. In the current study, we demonstrate the first bottom-up solution-processed two-terminal perovskite/perovskite tandem solar cell via developing a novel CRL: spiro-OMeTAD/PEDOT:PSS/PEI/PCBM:PEI. This CRL is efficient to collect electrons and holes at its top and bottom surfaces, and robust enough to protect the bottom perovskite film during the top perovskite film deposition. Moreover, the CRL is prepared by orthogonal solvent processing at low temperature, which is compatible with the pre-deposited perovskite film underneath. The PEI/PCBM:PEI is specially developed for efficient electron collection in both single-junction and tandem perovskite solar cells. With the optimized CRL to bridge the two CH3NH3PbI3 perovskite subcells, the tandem solar cell yields an open-circuit voltage (VOC) of up to 1.89 V that is close to the sum of the two perovskite subcells.

•A Semitransparent, non-fullerene, flexible all-plastic organic solar cell is fabricated.•Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) is used as the top and bottom electrodes.•The fullerene-free device shows better efficiency and better bending stability than the fullerene counterpart.Semitransparent, non-fullerene and flexible all-plastic organic solar cell (OSC) based on a blend of poly(3-hexylthiophene) (P3HT) and non-fullerene acceptor IDT-2BR is fabricated using conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as bottom electrode and film-transfer laminated PEDOT:PSS as top electrode. The solar cell shows average visible transmittance of ca. 50%, which can be potentially used for electricity-generating windows. The all-plastic device shows lower power conversion efficiency (PCE) of 2.88% than the traditional inverted device (4.2%), owing to much higher transmittance and lower conductivity of PEDOT:PSS. Our non-fullerene system P3HT:IDT-2BR shows better performance than fullerene system P3HT:PC61BM (PCE = 2.2%) in all-plastic OSCs. Furthermore, the non-fullerene OSC shows better bending stability than the fullerene counterpart.

Inverted planar perovskite solar cells using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the hole-transporting layer (HTL) are very attractive because of their low-temperature and easy processing. However, the planar cells with the PEDOT:PSS HTL typically display lower open-circuit voltage (VOC) (about 0.90 V) than that of devices with TiO2-based conventional structure (1.0–1.1 V). The underlying reasons are still not clear. In this work, we report the PEDOT:PSS that is intrinsically p-doped can be chemically reduced by methylamine iodide (MAI) and MAPbI3. The reaction reduces the work function (WF) of PEDOT:PSS, which suppresses the efficient hole collection and yields lower VOC. To overcome this issue, we adopt undoped semiconducting polymers that are intrinsically nonreduction-active (NRA) as the HTL for inverted planar perovskite solar cells. The cells display enhanced VOC from 0.88 ± 0.04 V (PEDOT:PSS HTL, reference cells) to 1.02 ± 0.03 V (P3HT HTL) and 1.04 ± 0.03 V (PTB7 and PTB-Th HTL). The power conversion efficiency (PCE) of the devices with these NRA HTL reaches about 17%.Keywords: hole-transporting layers; nonreduction-active; open-circuit voltage; PEDOT:PSS; perovskite solar cells;

Highly conductive polymer films on plastic substrates are desirable for the application of flexible electronics. Here, we report the conductivity of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) can be enhanced to 1460 S/cm via phosphoric acid (H3PO4) treatment. The conductivity enhancement is associated with the partial removal of PSS from the film. The H3PO4 treatment is compatible with plastic substrates, while sulfuric acid (H2SO4) can easily damage the plastic substrate. With the flexible electrode of poly(ether sulfone) (PES)/H3PO4-treated PEDOT:PSS, we have demonstrated flexible all-plastic solar cells (PES/H3PO4-treated PEDOT:PSS/PEI/P3HT:ICBA/EG-PEDOT:PSS). The cells exhibit an open-circuit voltage of 0.84 V, a fill factor of 0.60, and a power conversion efficiency of 3.3% under 100 mW/cm2 white light illumination.Keywords: all-plastic solar cells; conductivity; flexible electrode; PEDOT:PSS; phosphoric acid treatment;

Semitransparent solar cells are highly attractive for application as power-generating windows. In this work, we present semitransparent perovskite solar cells that employ conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film as the transparent counter electrode. The PEDOT:PSS electrode is prepared by transfer lamination technique using plastic wrap as the transfer medium. The use of the transfer lamination technique avoids the damage of the CH3NH3PbI3 perovskite film by direct contact of PEDOT:PSS aqueous solution. The semitransparent perovskite solar cells yield a power conversion efficiency of 10.1% at an area of about 0.06 cm2 and 2.9% at an area of 1 cm2. The device structure and the fabrication technique provide a facile way to produce semitransparent perovskite solar cells.Keywords: large-area proviskite solar cell; PEDOT:PSS counter electrode; perovskite solar cell; plastic wrap; semitransparent; transfer lamination technique

•A nonionic surfactant enhances both wetting property and conductivity of PEDOT:PSS.•The conductivity of PEDOT:PSS is enhanced up to 526 S/cm by the surfactant.•PEDOT:PSS with the surfactant is used as top electrode for vacuum-free solar cells.•The device based on P3HT:ICBA exhibit a FF of 60% and a PCE of 4.1%.In this work, we report a nonionic surfactant (polyethylene glycol 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol ether, PEG-TmDD) that can improve the wetting property of PEDOT:PSS aqueous solution on the organic photoactive layer and simultaneously enhance the electrical conductivity of PEDOT:PSS film up to 526 S/cm. Furthermore, the conductivity enhancement is significantly dependent on the thermal annealing, which is contrary to the conductivity behavior of PEDOT:PSS film prepared from the formulation added with ethylene glycol (EG) where the conductivity is almost independent of the thermal annealing. The temperature dependence of the conductivity of PEDOT:PSS by PEG-TmDD is possibly ascribed to decomposition of PEG-TmDD into EG and TmDD during thermal annealing. With the high conductivity and good wetting on the active layer, PEDOT:PSS mixed with PEG-TmDD is used as the top electrode for organic solar cells. The cells exhibit a fill factor of 60% and a power conversion efficiency of 4.1% using poly(3-hexylthiophene):indene-C60 bis-adduct as the active layer. The results indicate that the new formulation of PEDOT:PSS mixed with PEG-TmDD is suitable for preparing a top electrode for vacuum-free organic solar cells.

•PEI coating decreases conductivity of PEDOT:PSS films.•2-Methoxythanol enhances conductivity of pristine PH1000 film from 1 to 744 S/cm.•PEI coating increases the absorption of sulfuric acid treated PEDOT:PSS films.•Solar cells with simple structure of PEDOT-1/P3HT:ICBA/PEDOT-2 exhibit FF of 0.62.We report on conductivity and optical property of three different types of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) films [pristine PH1000 film (PH1000-p), with 5 wt.% ethylene glycol additive (PH1000-EG) and with sulfuric acid post-treatment (PH1000-SA)] before and after polyethylenimine (PEI) treatment. The PEI is found to decrease the conductivity of all the PEDOT:PSS films. The processing solvent of 2-methoxyethanol is found to significantly enhance the conductivity of PH1000-p from 1.1 up to 744 S/cm while the processing solvent of isopropanol or water does not change the conductivity of PH1000-p much. As for the optical properties, the PEI treatment slightly changes the transmittance and reflectance of PH1000-p and PH1000-EG films, while the PEI leads to an substantial increase of the absorptance in the spectral region of 400–1100 nm of the PH1000-SA films. Though the optical property and conductivity of the three different types of PEDOT:PSS films vary with the PEI treatment, the treated PEDOT:PSS films exhibit similar low work function. We demonstrate solar cells with a simple device structure of glass/low-WF PEDOT:PSS/P3HT:ICBA/high-WF PEDOT:PSS cells that exhibit good performance with open-circuit voltage of 0.82 V and fill factor up to 0.62 under 100 mW/cm2 white light illumination.

•Flexible hybrid cell can harvest the solar and mechanical energies simultaneously.•The flexible hybrid cell can power the wearable devices even in the weak light conditions.Flexible device that can harvest renewable energy from environment is urgently needed nowadays. A satisfactory device should be able to harvest multi-type energies around the clock without any economic difficulties for mass production. Here we report an all solution processed flexible hybrid cell by integrating an organic solar cell and triboelectric nanogenerator into a thin film, which is capable to convert both of the solar and mechanical energies into electric power independently or simultaneously, the generated energy can be used either to charge an energy storage unit or as a primary energy source for wearable self-powered devices even in the weak light conditions. This work provides a feasible and scalable method to fabricate the hybrid energy devices within reasonable cost to overcome the environmental restrictions of the devices that the mode of harvest single energy form.

Polyethylenimine (PEI) has been widely used to produce low-work-function electrodes. Generally, PEI modification is prepared by spin coating from 2-methoxyethanol solution. In this work, we explore the method for PEI modification on indium tin oxide (ITO) by dipping the ITO sample into PEI aqueous solution for organic solar cells. The PEI prepared in this method could reduce the work function of ITO as effectively as PEI prepared by spin coating from 2-methoxyethanol solution. H2O as the processing solvent is more environmentally friendly and much cheaper compared to the 2-methoxyethanol solvent. The dipping method is also compatible with large-area samples. With low-work-function ITO treated by the dipping method, solar cells with a simple structure of glass/ITO/PEI(dipping)/P3HT:ICBA/PEDOT:PSS(vacuum-free processing) display a high open-circuit voltage of 0.86 ± 0.01, a high fill factor of 66 ± 2%, and power conversion efficiency of 4.4 ± 0.3% under 100 mW/cm2 illumination.Keywords: dipping; low work function; organic solar cells; PEI aqueous solution

•Conducting polymer PEDOT:PSS film is prepared by transfer lamination.•Plastic wrap is used as the transfer medium for PEDOT:PSS transfer.•The transferred PEDOT:PSS used as the top electrode of organic solar cells.•Solar cells exhibit averaged FF of 0.60 and PCE of 4.0%.We report on the film preparation of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by transfer lamination using plastic wrap as the transfer medium. Comparing with the commonly used polydimethylsiloxane (PDMS) transfer medium, the plastic wrap is cheaper, easier to access and for mass production. The surface of plastic wrap is less hydrophobic than that of PDMS, aqueous PEDOT:PSS solution with 0.5 wt.% surfactant can wet the plastic wrap well. No plasma or ultraviolet ozone treatment is needed on the plastic wrap prior to the coating of PEDOT:PSS, while plasma treatment is necessary when PDMS is used transfer medium. That simplifies the fabrication process. Organic solar cells with the PEDOT:PSS top electrode transferred using plastic wrap transfer medium exhibit an averaged fill factor of 0.60 and an averaged power conversion efficiency of 4.0%, comparable to that of reference solar cells with PDMS as transfer medium for PEDOT:PSS transfer.Graphical abstract

Flexible and light-weight photovoltaics are desirable for applications that involve their integration with flexible electronics. In this study, we demonstrate efficient flexible perovskite solar cells with a novel device architecture (that is indium-tin-oxide (ITO) free and top-illuminated) using a low-temperature processed doped fullerene as the electron-transporting layer. Silver is used as the bottom electrode, and a transparent conducting polymer electrode is used as the top electrode for light illuminating through to the active layer. Stearyldimethylbenzylammonium chloride (SDBAC)-doped [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) was the electron-transporting layer wherein SDBAC could enhance the conductivity by three orders of magnitude and therefore enhance the solar cell performance. The ITO-free flexible perovskite solar cells display power conversion efficiency of 11.8% on a polyethersulfone substrate and can maintain 84% of the initial PCE after 1000 bending cycles at a bending radius of 6 mm.

The fabrication of thin layers of organic photoactive materials (typically ca. 100–200 nm thick) over large area is needed for the commercial realization of organic solar cells. This is challenging because defects on these thin layers can cause high leakage currents which lead to poor device performance and, ultimately, to poor device yield. Here, we report that organic solar cells with a tandem structure can display an increased tolerance to defects and are found less susceptible to parasitic area scaling-up effects compared to single-junction solar cells. We demonstrate 10.5 cm2 flexible tandem solar cells with a power conversion efficiency of 6.5% with a fabrication yield of over 90% in a laboratory environment. The high fabrication yield and good performance displayed by tandem organic solar cells suggest that despite their increased complexity, they could provide a viable path towards the commercial realization of efficient large-area organic solar cells.

Low dark current is critical to realize high-performance near-infrared organic photodetectors (NIR-OPDs). In general, organic photodetectors (OPDs) are with vacuum-deposited metals as the top electrode. The deposition of such metal would inevitably form doping to the organic active layer and thus yield high dark current. Herein, we employ transfer-printed conducting polymer (tp-CP) as the top electrode instead of the vacuum-deposited metal electrode. The photodetector with tp-CP electrode exhibits over two orders of magnitude lower dark current density than the device with the vacuum-deposited metal electrode. The photodetector with tp-CP electrode displays a responsivity of 0.37 A W−1 at 850 nm and a low dark current density of 3.0 nA cm−2 at −0.2 V based on a near-infrared (NIR) active layer of PMDPP3T:PC61BM that absorbs photons up to 1000 nm. The detectivity of the NIR photodetector reaches as high as over 1013 Jones. Furthermore, the NIR photodetector is double-side responsive to incident light, either from the bottom or the top electrode, because the top tp-CP electrode shows similar transparency as the bottom indium-tin oxide electrode.

Perovskite solar cells have been attracting a lot of attention because of their high power conversion efficiency and low-cost processing. However, device reproducibility has been a problem. The fabrication atmosphere is regarded as one of the possible reasons. So far, there has been a lack of direct evidence to prove which kind of atmosphere and how the atmosphere affects the device performance. Here, we report that the methylamine (MA, boiling point: −6 °C) that is used to synthesize the methylammonium iodide (MAI) could chemically reduce the PEDOT:PSS hole-transporting layer. After the reduction, a strong absorbance band appears at 400–1100 nm and the conductivity and work function simultaneously decrease. Furthermore, the MA-reduced PEDOT:PSS films are also found to be easily oxidized in air. The reduced work function of the PEDOT:PSS layer leads to poor hole collection and yields low open-circuit voltage, short-circuit current and power conversion efficiency of the perovskite solar cells. Therefore, though the MA vapor-containing fabrication atmosphere is beneficial to the performance of TiO2-based perovskite solar cells in which the bottom electrode collects the electrons, it is detrimental to that of PEDOT:PSS-based solar cells.

Building a tandem structure is an effective strategy to enhance the photovoltaic performance of solar cells. In the realization of a two-terminal tandem device, the charge recombination layer (CRL) plays an essential role. In the current study, we demonstrate the first bottom-up solution-processed two-terminal perovskite/perovskite tandem solar cell via developing a novel CRL: spiro-OMeTAD/PEDOT:PSS/PEI/PCBM:PEI. This CRL is efficient to collect electrons and holes at its top and bottom surfaces, and robust enough to protect the bottom perovskite film during the top perovskite film deposition. Moreover, the CRL is prepared by orthogonal solvent processing at low temperature, which is compatible with the pre-deposited perovskite film underneath. The PEI/PCBM:PEI is specially developed for efficient electron collection in both single-junction and tandem perovskite solar cells. With the optimized CRL to bridge the two CH3NH3PbI3 perovskite subcells, the tandem solar cell yields an open-circuit voltage (VOC) of up to 1.89 V that is close to the sum of the two perovskite subcells.