Solution-processed Solar Cells Research Articles

Enhancing PbS Colloidal Quantum Dot Tandem Solar Cell Performance by Graded Band Alignment.

Colloidal quantum dot solids are attractive candidates for tandem solar cells because of their widely tunable bandgaps. However, the development of the quantum dot tandem solar cell has lagged far behind that of its single-junction counterpart. One of the fundamental problems with colloidal quantum dot solar cells is the relatively small diffusion length, which limits the quantum dot absorbing layer thickness and hence the power conversion efficiency. In this research, guided by optical modeling and utilizing a graded band alignment strategy, a two-terminal monolithic solution-processed quantum dot tandem solar cell has been successfully fabricated and a power conversion efficiency of 6.8% has been achieved. The band grading approach utilized the complementary tuning of work functions and band alignment through judicious choices of the nanoparticle surface chemistry and quantum dot confined size. This work demonstrates a general approach to improving the efficiency for tandem thin-film solar cells.

Effect of sulfurization process on the properties of solution-processed Cu2SnS3 thin film solar cells

Cu2SnS3 (CTS) based thin film solar cells were successfully fabricated through a facile solution processes followed by a post-sulfurization treatment. The influence of sulfurization temperature and time on the structural, compositional, morphological, optical and also the electrical properties of the solar cells were systematically studied. The results showed that the CTS thin films sulfurized at 580 °C would be a void-free and densely packed absorber layer with the grain size to be microns, having the band gap of ~ 1.01 eV. Furthermore, the sulfurization time is the key factor to influence the electronic properties of CTS absorber layer, and thus resulting in the best device performance of PCE = 2.41% by 20 min sulfurization process.

Efficient, 23%, Solution-Processed Perovskite Tandem Cells

In this issue of Joule, Palmstrom and coworkers present efficient solution-processed two-terminal solar cells employing two metal halide perovskite-based absorbers. The key to this achievement is 2-fold: the deposition of a thin yet robust transparent conductor in between the two sub-cells allows solution processing of the back-cell and enables efficient and local charge recombination. Furthermore, the insertion of large cations reduces halide segregation and enables a higher open-circuit voltage and stability for the high-band-gap sub-cell.

Cold Crystallization Temperature Correlated Phase Separation, Performance, and Stability of Polymer Solar Cells

Summary Fabrication of non-fullerene (NF)-based polymer solar cells (PSCs) has been an arduous task involving various approaches to optimize the morphology of blend film for superior performance. In this work, unique exothermic peaks can be observed in differential scanning calorimetry first heating scan of the blend films, which are assigned to the cold crystallization temperature (TCC) of NF acceptors when blended with different degree of crystallinity polymers, and TCC contains the phase information at quenched stage of the film development. A linear correlation between the phase separation parameters and TCC was established, which helps to select the appropriate donor for a specific acceptor. Utilizing multiple polymer-NF blend systems, we report an expectation TCC value between 210°C and 230°C, which would promise balanced domain size, purity, and consequentially superior performance. Further investigation revealed that TCC can also be used to evaluate the thermal stability.

A dopant-free twisted organic small-molecule hole transport material for inverted planar perovskite solar cells with enhanced efficiency and operational stability

Low-cost solution-processable inverted perovskite solar cells (PSCs) demonstrate great potential toward future photovoltaic market. Unfortunately, general hole transport materials (HTMs) within inverted structure make the performance and stability far uncompetitive compared to the normal structure. Interrogating the fundamentals of these materials, moderate charge carrier mobility and susceptible environmental stability of the undoped molecules are the main causes. Herein, a twisted molecule XY1 is developed as a potential robust dopant-free HTM for inverted PSCs. Compared with traditional poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), XY1 exhibits much higher hole transport ability and peculiar ultraviolet absorption. Hysteresis-free inverted PSCs based on XY1 exhibits the power conversion efficiency (PCE) of 18.78%, which is among the highest values for inverted PSCs based on dopant-free HTMs. After light soaking for 200 h, the original PCE of XY1-based device is still maintained at the 95% level, indicating the substantially improved operational stability. Besides, large-area (1.00 cm2) inverted PSC based on XY1 shows a competitive PCE of 17.82%.

Multiple Roles of Cobalt Pyrazol-Pyridine Complexes in High-Performing Perovskite Solar Cells.

Chemical doping is a ubiquitously applied strategy to improve the charge-transfer and conductivity characteristics of spiro-OMeTAD, a hole-transporting material (HTM) used widely in solution-processed perovskite solar cells (PSCs). Cobalt(III) complexes are commonly employed HTM dopants, whose major role is to oxidize spiro-OMeTAD to providep-doping for improved conductivity. The present work discloses additional, previously unknown important functions of cobalt complexes in the HTM films that influence the photovoltaic performance. Specifically, it is demonstrated that commercialp-dopant FK269 (bis(2,6-di(1H-pyrazol-1-yl)pyridine) cobalt(III) tris(bis(trifluoromethylsulfonyl)imide)) reduces the interfacial recombination and alleviates the decomposition of the perovskite layerunder the action oftert-butylpyridine and lithium bis(trifluoromethanesulfonyl)imide. These effects are demonstrated for 1 cm2 perovskite solar cells that achieve a stabilized power conversion efficiency of 19% under 1 sun irradiation.

Identifying inverted-hysteresis behavior of CH3NH3PbI3−xClx planar hybrid perovskite solar cells based on external bias precondition

Solution-processable hybrid perovskite solar cells offer potential in the photovoltaic field due to their low-cost fabrication and high efficiency. However, an undesirable current–voltage (J-V) hysteresis hampers the applications of perovskite solar cells. In particular, for the inverted device, the understandings for J-V hysteresis origination are not uniform, and the inverted hysteresis phenomenon has been further complicated the hysteresis behavior. In this report, an external bias precondition method is adopted to unveil the origin of the inverted hysteresis. The results indicate that the extents of inverted hysteresis are very much dependent on the bias direction of the precondition. To further unveil the effect of the precondition on inverted hysteresis, the microscopic J-V hysteresis was also observed by using conductive atomic force microscopy (c-AFM) measurements. The results indicate that ion migration and accumulation slowly built up at the grain boundaries of the perovskite film when repeating the scan using c-AFM. Furthermore, the transient characteristics based on capacity-frequency plots and open-circuit voltage decay further identify the presence of difference for the different bias preconditions on the device, because the different bias precondition could induce different directions of ions migration and accumulation. These observations are surprising; it can be further identified that the inverted hysteresis originates from the ionic migration and accumulation, and the grain boundary is as the channel of ionic migration and accumulation. Therefore, the grain boundary plays an important role on the hysteresis effect, and preparation of large-grain or single-crystal perovskite films is the way to reduce the hysteresis effect.

A 9.16% Power Conversion Efficiency Organic Solar Cell with a Porphyrin Conjugated Polymer Using a Nonfullerene Acceptor.

A new low-molecular-weight porphyrin-based polymer, PPPyDPP, with pyridine-capped diketopyrrolopyrrole (DPP) has been synthesized, and its optical and electrochemical properties were investigated. The polymer is prepared with a low content of homocoupling units and gives a widely spread absorption from 400 to 900 nm with a narrow optical band gap of 1.46 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are respectively located at -5.27 and -3.78 eV, respectively. PPPyDPP was used as the electron donor, whereas [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and bis(rhodanine)indolo-[3,2-b]-carbazole (ICzRd2), a nonfullerene small molecule, were used as acceptors for the fabrication of solution-processed bulk heterojunction polymer solar cells. Overall power conversion efficiencies (PCEs) of 7.31 and 9.16% (record high for porphyrin-containing polymers) were obtained for PC71BM and ICzRd2, respectively. A high Voc of 1.01 V and a low Eloss of 0.45 eV may explain this new record.

Quantum Dot Donor-Polymer Acceptor Architecture for a FRET-Enabled Solar Cell.

Forster resonance energy-transfer (FRET)-based solution-processed solar cell is fabricated with cadmium sulfide (CdS) as the energy donor and poly[ N-9'-heptadecanyl-2,7-carbazole- alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) as the energy acceptor. Carbon dots (C-dots) deposited on carbon fabric are applied as a counter electrode. Although electron injection from CdS to PCDTBT is energetically disfavored, evidences for energy transfer between the two components of the cell are obtained in terms of FRET parameters with the relative quantum yield of donor CdS quantum dots (QDs) being ∼0.3, a Forster radius of ∼3.7 nm, and an energy-transfer efficiency of ∼55%. Power conversion efficiency (PCE) of the TiO2/PCDTBT cell without the donor is 0.23% and when coupled with donor CdS QDs, the ensuing TiO2/PCDTBT/CdS cell experiences a 23 time increment in PCE, reaching 5.3%. The complete FRET cell: TiO2/PCDTBT/CdS/ZnS-S2--C-dots/C-fabric produces a PCE of 7.42%, under 1 sun illumination. External quantum efficiency studies reveal an enhanced spectral response spanning from 300 to 670 nm, with 300 and 175% increases attained for the FRET-enabled TiO2/PCDTBT/CdS/ZnS photoanode compared with the TiO2/PCDTBT photoanode over the blue and green-red portions of the solar spectrum.

(Invited) Designing Novel Organic Small Molecular As Electron Transport Layers for Planar Perovskite Solar Cells

Compared to the traditional-architecture perovskite photovoltaics (n-i-p type), which use metal oxide as electron transport layers(ETLs) and organic semiconducting materials as hole transport layers, the fabrication of metal-oxide-free, solution-processed inverted perovskite solar cells (PSCs) is more desired because of low-temperatures and all-solution-based applications in future commercial PSC modules. In a typical configuration of inverted PSCs, the widely used ETL compound is the fullerene-based phenyl-C61-butyric acid methyl ester (PCBM), which currently is the best organic ETL material. The cost of this compound is very high, and the morphology and electrical properties are very sensitive to experimental conditions. In this talk, I will talk the recent progress of new organic ETL materials in my group for the replacement of PCBM in inverted PSCs. We believe that easily-accessible simple n-type small molecules are promising ETL candidates to further propel inverted PSCs to practical applications. References Ning Wang, Kexiang Zhao, Tao Ding, Wenbo Liu, Ali Said Ahmed, Zongrui Wang, Miaomiao Tian, Xiao Wei Sun,* Qichun Zhang*, “Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer”, Adv. Energy Mater. 2017, DOI: 10.1002/aenm.201700522.Pei-Yang Gu, Ning Wang, Chengyuan Wang, Yecheng Zhou, Guankui Long, Miaomiao Tian, Wangqiao Chen, Xiao Wei Sun,* Mercouri G. Kanatzidis*, Qichun Zhang,* “Pushing up the efficiency of planar perovskite solar cells to 18.2% with organic small molecular as electron transport layer”, J. Mater. Chem. A. 2017, 5, 7339 – 7344.Pei-Yang Gu, Ning Wang, Anyang Wu, Zilong Wang, Miaomiao Tian, Zhisheng Fu,* Xiao Wei Sun,* Qichun Zhang* “Azaacene Derivative as Promising Electron Transport Layer for Inverted Perovskite Solar Cells”, Chem. Asian J. 2016, 11(15), 2135–2138.

Open atmospheric processed perovskite solar cells using dopant-free, highly hydrophobic hole-transporting materials: Influence of thiophene and selenophene π-spacers on charge transport and recombination properties

We have synthesized three donor-π-acceptor polymers by crosslinking 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b'] and pyrrolo[3,4-c]pyrrole-1,3-dione with thiophene, thieno[3,2-b]thiophene, selenophene as π-spacer. These polymers have been integrated successfully as dopant-free hole-transporting materials in fully open atmosphere solution-processed lead halide perovskite solar cells. Due to the synergistic impact of linear and long alkyl chains with different π-spacers, these hole transporters retard interfacial charge recombination and afford rich solubility and morphology resulting long carrier lifetime with uniform film coverage. In addition, these hole transporters possess a deeper highest occupied molecular orbitals (−5.40, −5.37 and −5.42 eV) and suitable over potential with perovskite valence band (−5.43 eV), favorable for efficient hole extraction. Further, the large number of S-based heterocycles helps to strengthen the interaction of the perovskite and hole transporting layer interface and high hydrophobicity (∼104°) facilitate enhanced stability with insignificant hysteresis. As a result, the power conversion efficiency has reached 14% under AM1.5 G without any additives in fully open atmospheric conditions. Our findings highlight the better energy transfer capabilities of additive-free hole transporters as a promising candidate for developing stable PSCs in the open air.

Highly efficient solution-processed CZTSSe solar cells based on a convenient sodium-incorporated post-treatment method

Abstract In CZTSSe solar cells, a simple sodium-incorporation post-treatment method toward solution-processed Cu2ZnSnS4 precursor films is presented in this work. An ultrathin NaCl film is deposited on Cu2ZnSnS4 precursor films by spin-coating NaCl solution. In subsequent selenization process, the introduction of NaCl is found to be benefacial for the formation of Cu2-xSe, which can further facilitate the element transportation, leading to dense and smooth CZTSSe films with large grains and less impurity Cu2Sn(S,Se)3 phase. SIMS depth profiles confirm the gradient distribution of the sodium element in Na-doped absorbers. Photoluminescence spectra show that the introduction of appropriate sodium into the absorber can inhibit the band tail states. As high as 11.18% of power conversion efficiency (PCE) is achieved for the device treated with 5 mg mL−1 NaCl solution, and an average efficiency of Na-doped devices is 10.71%, 13% higher than that of the control groups (9.45%). Besides, the depletion width and the charge recombination lifetime can also have regular variation with sodium treatment. This work offers an easy modification method for high-quality Na-doped CZTSSe films and high-performance devices, in the meantime, it can also help to further understand the effects of sodium in CZTSSe solar cells.

The Effects of ZnTe:Cu Back Contact on the Performance of CdTe Nanocrystal Solar Cells with Inverted Structure.

CdTe nanocrystal (NC) solar cells have received much attention in recent years due to their low cost and environmentally friendly fabrication process. Nowadays, the back contact is still the key issue for further improving device performance. It is well known that, in the case of CdTe thin-film solar cells prepared with the close-spaced sublimation (CSS) method, Cu-doped CdTe can drastically decrease the series resistance of CdTe solar cells and result in high device performance. However, there are still few reports on solution-processed CdTe NC solar cells with Cu-doped back contact. In this work, ZnTe:Cu or Cu:Au back contact layer (buffer layer) was deposited on the CdTe NC thin film by thermal evaporation and devices with inverted structure of ITO/ZnO/CdSe/CdTe/ZnTe:Cu (or Cu)/Au were fabricated and investigated. It was found that, comparing to an Au or Cu:Au device, the incorporation of ZnTe:Cu as a back contact layer can improve the open circuit voltage (Voc) and fill factor (FF) due to an optimized band alignment, which results in enhanced power conversion efficiency (PCE). By carefully optimizing the treatment of the ZnTe:Cu film (altering the film thickness and annealing temperature), an excellent PCE of 6.38% was obtained, which showed a 21.06% improvement compared with a device without ZnTe:Cu layer (with a device structure of ITO/ZnO/CdSe/CdTe/Au).

Small molecule acceptors with indacenodithiophene–benzodithiophene–indacenodithiophene as donating cores for solution-processed non-fullerene solar cells

Abstract Two acceptor-donor1-donor2-donor1-acceptor (A-D1-D2-D1-A)-type narrow bandgap small molecules BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 with indacenodithiophene–benzodithiophene-indacenodithiophene (BDT-(IDT)2) as donating cores and 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2FIC) as end groups, have been synthesized and investigated as non-fullerene acceptors in solution-processed polymer solar cells (PSCs). The optical bandgaps of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 are 1.59 eV and 1.54 eV, respectively. Using PBDB-T as donor, the power conversion efficiencies (PCEs) of 4.98% and 6.21% are achieved by PSCs based on BDT(IDT-IC)2 and BDT(IDT-IC-2F)2, respectively.

4-Tert butylpyridine induced MAPbI3 film quality enhancement for improving the photovoltaic performance of perovskite solar cells with two-step deposition route

As we known, high-efficiency of the solution-processed perovskite solar cells can be achieved by modulating the perovskite film quality. Here, the prevalent 4-tert-butylpyridine is firstly utilized as an additive of CH3NH3I solution for improving the CH3NH3PbI3 film quality fabricated with two-step deposition method. Due to the strong polarity and interaction between N atom on TBP and Pb atom on CH3NH3PbI3, this strategy leads to a substantial enhancement of the CH3NH3PbI3 film quality with larger grains, higher crystallinity and more complete coverage on the substrate than that of the pristine one. Thereby, the TBP-based CH3NH3PbI3 film simultaneously exhibits superior optical properties and surface hydrophobicity. By optimization, the perovskite solar cells fabricated with TBP addition at a volume ratio of 7/1000 (v/v, TBP/isopropanol) exhibits significant performance enhancement with a power conversion efficiency reaching 17.06%, while the pristine one merely obtains a lower value of 14.56%. More importantly, this type of cells also shows a desirable efficiency reproducibility and long-time stability in air atmosphere. In conclusion, our work demonstrates another meaningful application for the TBP additive and provides a facile and effective strategy for further improving the performance of perovskite solar cells.

Universal Nanoparticle Wetting Agent for Upscaling Perovskite Solar Cells.

Solution-processed perovskite solar cells reach efficiencies over 23% on lab-scale. However, a reproducible transfer of these established processes to upscaling techniques or different substrate surfaces requires a highly controllable perovskite film formation. Especially, hydrophobic surfaces cause severe dewetting issues. Such surfaces are particularly crucial for the so-called standard n-i-p cell architecture when fullerene-based electron transport layers are employed underneath perovskite absorber films. In this work, a unique and universally applicable method was developed based on the deposition of size-controlled Al2O3 or SiO2 nanoparticles. By enhancing the surface energy, they act as a universal wetting agent. This allows perovskite precursor solutions to be spread perfectly over various substrates including problematic hydrophobic Si-wafers or fullerene self-assembled monolayers (C60-SAMs). Moreover, the results show that the perovskite morphology, solar cell performance, and reproducibility benefit from the presence of the nanoparticles at the interface. When applied to 144cm2 C60-SAM-coated substrates, homogenous coverage can be realized via spin coating resulting in average efficiencies of 16% (maximum 18%) on individualized cells with 0.1 cm2 active area. Modules in the same setup reached maximum efficiencies of 11 and 7% on 2.8 and 23.65cm2 aperture areas, respectively.

Solution-Processed All-Perovskite Multi-junction Solar Cells

Summary Multi-junction device architectures can increase the power conversion efficiency (PCE) of photovoltaic (PV) cells beyond the single-junction thermodynamic limit. However, these devices are challenging to produce by solution-based methods, where dissolution of underlying layers is problematic. By employing a highly volatile acetonitrile(CH3CN)/methylamine(CH3NH2) (ACN/MA) solvent-based perovskite solution, we demonstrate fully solution-processed absorber, transport, and recombination layers for monolithic all-perovskite tandem and triple-junction solar cells. By combining FA0.83Cs0.17Pb(Br0.7I0.3)3 (1.94 eV) and MAPbI3 (1.57 eV) junctions, we reach two-terminal tandem PCEs of more than 15% (steady state). We show that a MAPb0.75Sn0.25I3 (1.34 eV) narrow band-gap perovskite can be processed via the ACN/MA solvent-based system, demonstrating the first proof-of-concept, monolithic all-perovskite triple-junction solar cell with an open-circuit voltage reaching 2.83 V. Through optical and electronic modeling, we estimate the achievable PCE of a state-of-the-art triple-junction device architecture to be 26.7%. Our work opens new possibilities for large-scale, low-cost, printable perovskite multi-junction solar cells.

Review of lead-free halide perovskites as light-absorbers for photovoltaic applications: From materials to solar cells

Metal halide perovskite materials have revolutionized the solution-processed solar cells and become the vanguard of research focus with an unprecedented improvement of power conversion efficiencies up to 23.3%, which pose a remarkable challenge to thin film and multicrystalline silicon photovoltaics. Nevertheless, for conventional perovskite solar cells based on lead, it is ineluctable to take the toxicity of lead and the long-term stability of the devices into consideration when the deployment of this technology in mass production is put on the agenda. To tackle the toxicity concern, extensive research effort has been devoted to the development of environmentally friendly lead-free halide perovskites for photovoltaic applications. Herein, we review the advancements in this emerging research area and discuss potential alternative elements to replace lead within the halide perovskite lattice. Particular attention will be focused on the properties of the corresponding perovskites and the relevant production of high efficiency photovoltaic devices, with the aim of identifying the materials to be competitive with lead-based perovskites. For the moment, tin-based perovskites appear likely to be the most preeminent from the point view of power conversion efficiency, despite the stabilities of the materials and the related devices are far from satisfaction. The introductions of germanium, bismuth, antimony, and certain transitional metals into perovskites brought about promising optoelectronic properties as well. This booming class of compounds constitutes unexplored territory and emerges as an affordable and robust candidate for green solar technology.

Solution-processed intermediate-band solar cells with lead sulfide quantum dots and lead halide perovskites

The intermediate-band solar cell (IBSC) with quantum dots and a bulk semiconductor matrix has potential for high power conversion efficiency, exceeding the Shockley-Queisser limit. However, the IBSCs reported to date have been fabricated only by dry process and their efficiencies are limited, because their photo-absorption layers have low particle density of quantum dots, defects due to lattice strain, and low bandgap energy of bulk semiconductors. Here we present solution-processed IBSCs containing photo-absorption layers where lead sulfide quantum dots are densely dispersed in methylammonium lead bromide perovskite matrices with a high bandgap energy of 2.3 eV under undistorted conditions. We confirm that the present IBSCs exhibit two-step photon absorption via intermediate-band at room temperature by inter-subband photocurrent spectroscopy.

Understanding the loss mechanisms in high-performance solution-processed small molecule bulk heterojunction solar cells doped with a PFN impurity.

Contamination of the active layer with an impurity could result in significant degradation in the performance of bulk heterojunction (BHJ) solar cells as a result of enhancing the loss of the charge carriers via a trap-assisted recombination. In this study, PFN as an impurity was intentionally introduced to a BHJ solar cell composed of a high-performance solution-processed small molecule (p-DTS(FBTTh2)2 as a donor and PC60BM as an acceptor. The power conversion efficiency (PCE) of PFN doped devices degrades owing to the reduction of short-circuit current (Jsc) and fill factor (FF). At a low concentration, PFN mostly reduces the generation of charge carriers, whereas doubling the PFN concentration conversely affects both generation and collection of charge carriers. Charge carrier dynamics of devices has also been probed using photovoltage decay, time-resolved charge extraction (TRCE) and photoinduced charge extraction by linearly increasing voltage (photo-CELIV) before and after incorporation of PFN. The results reveal that traps introduced by PFN reduce the decay of charge carriers via bimolecular recombination, leading to a higher charge carrier density and photovoltage at long times under an open-circuit potential (Voc). However, under short-circuit (Jsc) conditions, traps considerably impede the collection of charge carriers causing the appearance of an S-shaped current density-voltage curve.