Interface Engineering in Inorganic-Absorber Nanostructured Solar Cells.

Colloidal quantum dot (CQD) solar cells have attracted great interest due to their low cost and superior photo-electric properties. Remarkable improvements in cell performances of both quantum dot sensitized solar cells (QDSCs) and PbX (X = S, Se) based CQD solar cells have been achieved in recent years, and the power conversion efficiencies (PCEs) exceeding 12% were reported so far. In this review, we will focus on the recent progress in CQD solar cells. We firstly summarize the advance of CQD sensitizer materials and the strategies for enhancing carrier collection efficiency in QDSCs, including developing multi-component alloyed CQDs and core-shell structured CQDs, as well as various methods to suppress interfacial carrier recombination. Then, we discuss the device architecture development of PbX CQD based solar cells and surface/interface passivation methods to increase light absorption and carrier extraction efficiencies. Finally, a short summary, challenge, and perspective are given.

While hole extraction is crucial for the external quantum efficiency of conventional n-i-p colloidal quantum dot (CQD) solar cells (CQDSCs), sulfur-passivated p-type CQDs (pCQDs) have been the best hole-transport material (HTM) to date. In this work, we developed organic π-conjugated polymers (π-CPs) that can achieve substantially improved HTM performance compared with conventional pCQDs. A weakly electron-withdrawing triisopropylsilylethynyl (TIPS) group was employed with a weak donor moiety, benzo[1,2-b:4,5:b′]-dithiophene (BDT), in the push–pull structured π-CPs to optimize the optoelectronic properties of the HTM. The CQDSCs using TIPS-containing π-CPs achieved a PCE (13.03%) substantially higher than those previously reported using pCQD (11.33%) or π-CPs (11.25%) owing to the improved charge collection efficiency near the photoactive CQD layer/HTM interface. To the best of our knowledge, our CQDSCs using TIPS-based π-CPs achieved the highest reported PCE among SSE-free CQDSCs.

Utilizing light trapping interference effects in microcavity structured colloidal quantum dot solar cells: A combined theoretical and experimental approach

For the first time, the plasmonic gold bipyramids (Au BPs) are introduced to the PbS colloidal quantum dot (CQD) solar cells for improved infrared light harvesting. The localized surface plasmon resonance peaks of Au BPs matches perfectly with the absorption peaks of conventional PbS CQDs. Owing to the geometrical novelty of Au BPs, they exhibit significantly stronger far‐field scattering effect and near‐field enhancement than conventional plasmonic Au nanospheres (NSs). Consequently, device open‐circuit voltage (Voc) and short‐circuit current (Jsc) are simultaneously enhanced, while plasmonic photovoltaic devices based on Au NSs only achieve improved Jsc. The different effects and working mechanisms of these two Au nanoparticles are systematically investigated. Moreover, to realize effective broadband light harvesting, Au BPs and Au NSs are used together to simultaneously enhance the device optical and electrical properties. As a result, a significantly increased power conversion efficiency (PCE) of 9.58% is obtained compared to the PCE of 8.09% for the control devices due to the synergistic effect of the two plasmonic Au nanoparticles. Thus, this work reveals the intriguing plasmonic effect of Au BPs in CQD solar cells and may provide insight into the future plasmonic enhancement for solution‐processed new‐generation solar cells.

Sputtered transparent electrodes for optoelectronic devices: Induced damage and mitigation strategies

Semiconductor quantum dots (QDs) have the potential to design high efficiency solid-state QDSCs and ETA (Extremely Thin Absorber) solar cells. Tunable band gap and band edge through size quantization, large intrinsic dipole moments, and high extinction coefficients make them suitable light absorbing materials. A number of semiconductors have been explored in the design of solid state solar cells. These include CdS, CdSe, In2S3, and Sb2S3. Of particular interest is Sb2S3 which has a band gap of 1.7 − 1.8 eVallowing for light absorption throughout the visible range. Organometal perovskites such as CH3NH3PbI3 are the new semiconducting materials that have led to advances in thin film solar cells. However, hole transport remains a limiting factor in maximizing power conversion efficiency.Using Sb2S3 (absorber) and CuSCN (hole conductor), we have constructed ETA solar cells exhibiting a power conversion efficiency of 3.3%. We have employed transient absorption spectroscopy to probe the hole transfer between Sb2S3 and CuSCN. In the Sb2S3 absorber layer, photogenerated holes are rapidly localized on the sulfur atoms of the crystal lattice, forming a sulfide radical (S−•) species. This trapped hole is transferred from the Sb2S3 absorber to the CuSCN hole conductor with an exponential time constant of 1680 ps. Similarly the hole transport properties in (CH3NH3PbI3) and hole conductor copper iodide has been probed by impedence spectroscopy. Charge separation in Sb2S3 and (CH3NH3PbI3) based solar cells which provide new insight into the design of new architectures for higher efficiency devices will be discussed. ACKNOWLEDGMENT. The research described herein was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, United States Department of Energy through grant number DE-FC02-04ER15533.

Chapter 6 - Solution-processed quantum dot-sensitized solar cell based on “green” materials

In the case of cadmium sulfide (CdS) and cadmium selenide (CdSe)-based quantum dot-sensitized solar cells (QDSSCs), the addition of a zinc sulfide (ZnS) passivation layer improves the solar cell performance. In this study, multilayered QDSSCs were fabricated using CdS and CdSe quantum dots prepared by successive ionic layer adsorption and reaction (SILAR) method. The optimized QDSSCs were used to study the passivation effect of zinc chalcogenide layers: ZnS, zinc selenide (ZnSe), and zinc telluride (ZnTe). The best performing solar cell prepared from four SILAR cycles of CdS followed by six SILAR cycles of CdSe were used for subsequent deposition of Zn chalcogenide layers. It was observed that capping with ZnSe or ZnTe layer on the multilayered Cd chalcogenide QDs did not improve the solar cell performance. Only the addition of ZnS layer contributed to the better performance of the solar cell. The efficiency obtained in the optimized multilayered CdS/CdSe QDSSC with ZnS layer was 1.37 %, while the QDSSC with ZnSe or ZnTe capping showed lower performance. The behavior of the solar cells is explained with electrochemical impedance spectroscopy study.

Recent technological advances in conventional planar and microstructured solar cell architectures have significantly boosted the efficiencies of these devices near the corresponding theoretical values. Nanomaterials and nanostructures have promising potential to push the theoretical limits of solar cell efficiency even higher using the intrinsic advantages associated with these materials, including efficient photon management, rapid charge transfer, and short charge collection distances. However, at present the efficiency of nanostructured solar cells remains lower than that of conventional solar devices due to the accompanying losses associated with the employment of nanomaterials. The concurrent design of both optical and electrical components will presumably be an imperative route toward breaking the present‐day limit of nanostructured solar cells. This review summarizes the losses in traditional solar cells, and then discusses recent advances in applications of nanotechnology to solar devices from both optical and electrical perspectives. Finally, a rule for nanostructured solar cells by concurrently engineering the optical and electrical design is devised. Following these guidelines should allow for exceeding the theoretical limit of solar cell efficiency soon.

In spite of the promising design and architecture, quantum dot sensitized solar cells (QDSSCs) have a long way to go before they attain the actual projected photoconversion efficiencies. Such an inferior performance displayed by QDSSCs is primarily because of many unwanted recombination losses of charge carriers at various interfaces of the cell. Electron recombination due to back electron transfer at the photoanode/electrolyte interface is an important one that needs to be addressed, to improve the efficiency of these third generation nanostructured solar cells. The present work highlights the importance of conformal coverage of CdS quantum dots (QDs) on the surface of the nanocrystalline titania photoanode in arresting such recombinations, leading to improvement in the performance of the cells. Using the successive ionic layer adsorption and reaction (SILAR) process, photoanodes are subjected to different amounts of CdS QD sensitization by varying the number of cycles of deposition. The sensitized electrodes are characterized using UV–visible spectroscopy, cyclic voltammetry and transmission electron microscopy to evaluate the extent of surface coverage of titania electrodes by QDs. Sandwich solar cells are then fabricated using these electrodes and characterized employing electrochemical impedance spectroscopy and J–V characteristics. It is observed that maximum solar cell efficiency is obtained for photoanodes with conformal coating of QDs and any further deposition of sensitizer leads to QD aggregation and so reduces the performance of the solar cells.

With the global demand for renewable energy sources, there is a race in the scientific community to build a highly efficient solar cell. Nanostructured solar cells are a class of photovoltaic devices which is a strong candidate for high efficiency energy generation. Hot carrier solar cells are a kind of nanostructured solar cells, for offering high power conversion efficiency, compared with the conventional single absorber solar cell, through the process of extracting power from photo-generated high energetic carriers prior to thermalization to lattice via phonons. In this paper, we have analyzed the band structure, thermodynamic properties, and hot carrier transport in solar cell.

Single-walled carbon nanotubes (SWCNTs) show great potential as an alternative material for front electrodes in photovoltaic applications, especially for flexible devices. In this work, a press-transferred transparent SWCNT film was utilized as front electrode for colloidal quantum dot solar cells (CQDSCs). The solar cells were fabricated on both glass and flexible substrates, and maximum power conversion efficiencies of 5.5 and 5.6 %, respectively, were achieved, which corresponds to 90 and 92 % of an indium-doped tin oxide (ITO)-based device (6.1 %). The SWCNTs are therefore a very good alternative to the ITO-based electrodes especially for flexible solar cells. The optical electric field distribution and optical losses within the devices were simulated theoretically and the results agree with the experimental results. With the optical simulations that were performed it may also be possible to enhance the photovoltaic performance of SWCNT-based solar cells even further by optimizing the device configuration or by using additional optical active layers, thus reducing light reflection of the device and increasing light absorption in the quantum dot layer.

Colloidal quantum dots (CQDs) are semiconductor nanocrystals synthesized with, processed in, and deposited from the solution phase, potentially enabling low-cost, facile manufacture of solar cells. Unfortunately, CQD solar cell reports, until now, have only explored batch-processing methods—such as spin-coating—that offer limited capacity for scaling. Spray-coating could offer a means of producing uniform colloidal quantum dot films that yield high-quality devices. Here, we explore the versatility of the spray-coating method by producing CQD solar cells in a variety of previously unexplored substrate arrangements. The potential transferability of the spray-coating method to a roll-to-roll manufacturing process was tested by spray-coating the CQD active layer onto six substrates mounted on a rapidly rotating drum, yielding devices with an average power conversion efficiency of 6.7%. We further tested the manufacturability of the process by endeavoring to spray onto flexible substrates, only to find that spraying while the substrate was flexed was crucial to achieving champion performance of 7.2% without compromise to open-circuit voltage. Having deposited onto a substrate with one axis of curvature, we then built our CQD solar cells onto a spherical lens substrate having two axes of curvature resulting in a 5% efficient device. These results show that CQDs deposited using our spraying method can be integrated to large-area manufacturing processes and can be used to make solar cells on unconventional shapes.

A quantum dot‐sensitized solar cell (QD‐SSC) is one of these new type solar cells because of their advantages such as the inverse Auger effect, the tuning the particle size. Many quantum dot semiconductors such as CdS, CdSe, PbS and PbSe are used as the sensitizers instead of the ruthenium dye molecules in the QD‐SSC. The light absorbance of the quantum dot materials is changed as the dipping cycle and the amount of the materials. Therefore, the light absorbance and the efficiency of the QD‐SSC can be increased by using these characteristics.In this study, we tried to widen the light absorbance of the QD‐SSC by changing the doping amounts of Pb and Zn in the (Pb1‐xZnx)S material. Finally, we fabricated the (Pb1‐xZnx)S tandem structured QD‐SSC. When the tandem structure multilayer as the layer which contains less Zn amount is placed on the bottom and the layer which contains more Pb amount is placed on the top side, the light absorbance is broaden effectively by using this tandem structure. As a result, we got the I‐V characteristics of (Pb1‐xZnx)S QD‐SSC with 3 multilayer, 0.38 V, 3.219 mA/cm2, 0.39 of FF, 0.48%. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The application of inverse titania opals in nanostructured solar cells