wangzhaowei

Organic–inorganic hybrid perovskite solar cells based on CH₃NH₃PbI₃ have achieved great success with efficiencies exceeding 20%. However, there are increasing concerns over some reported efficiencies as the cells are susceptible to current–voltage (I–V) hysteresis effects. It is therefore essential that the origins and mechanisms of the I–V hysteresis can clearly be understood to minimize or eradicate these hysteresis effects completely for reliable quantification. Here, a detailed electro-optical study is presented that indicates the hysteresis originates from lingering processes persisting from sub-second to tens of seconds. Photocurrent transients, photoluminescence, electroluminescence, quasi-steady state photoinduced absorption processes, and X-ray diffraction in the perovskite solar cell configuration have been monitored. The slow processes originate from the structural response of the CH₃NH₃PbI₃ upon E-field application and/or charge accumulation, possibly involving methylammonium ions rotation/displacement and lattice distortion. The charge accumulation can arise from inefficient charge transfer at the perovskite interfaces, where it plays a pivotal role in the hysteresis. These findings underpin the significance of efficient charge transfer in reducing the hysteresis effects. Further improvements of CH₃NH₃PbI₃-based perovskite solar cells are possible through careful surface engineering of existing TiO₂ or through a judicious choice of alternative interfacial layers.

Electric-field and charge accumulation-induced perovskite structure response are used to explain the hysteresis and ultraslow dynamics in CH₃NH₃PbI₃ (MAPbI₃) perovskite-based solar cells. The charge transfer efficiency at perovskite interfaces is found to be significant in determining the severity of the hysteresis in perovskite photovoltaic devices. The interface between perovskite and TiO­₂ should be modified to minimize the hysteresis.

Three structurally different low molecular weight diketopyrrolopyrroles (DPPs) are synthesized in order to provide donors with a precise offset in their energy levels. The DPPs are characterized for optical, electrochemical, and thermal properties. By changing the terminal aryl groups attached to the DPP core from phenyl over m-pyridine to p-pyridine, different solid state packing is observed in thin film studies using UV/VIS absorption spectra and X-ray diffraction. Most importantly it is shown that both, reduction as well as oxidation potentials can be precisely tuned with a gradual stepping of about 100 meV by changing the terminal groups attached to the DPP core. Exploiting this energy level modification, these materials are tested in planar cascade organic photovoltaic devices using C₆₀ as acceptor. A sub nm thick interlayer of a suitable DPP derivative is introduced to obtain a distinct energy level cascade at the donor/acceptor interface. Power conversion efficiency as well as short-circuit current density is doubled with respect to the reference bilayer devices lacking the interface cascade. Spectrally resolved analysis of external quantum efficiency reveals that this enhancement can mainly be attributed to destabilization of bound charge transfer states formed in the C₆₀ layer at the interlayer interface, thus reducing geminate recombination losses.

Tailored low molecular weight diketopyrrolopyrrole compounds with a precise energy level offset are synthesized by tuning the electron deficiency on the terminal aryl unit. Application of these compounds in cascade solar cells in combination with C₆₀ leads to drastically increased short-circuit current densities and power conversion efficiencies.

Interface disorder comprehensively explains the up-and-down (or vice versa) energy shift commonly measured for molecular donor–acceptor interfaces including those used in organic photovoltaics. Full characterization of such interfaces with photoelectron spectroscopy and complementary electrostatic calculations reveal that electronic tail states, induced by disorder, lead to the complex energy level alignment observed.

Whereas the role of molecularly mixed domains in organic photovoltaic devices for charge generation is extensively discussed in the literature, the impact on charge recombination and thus fill factor is largely unexplored. Here, a combination of soft X-ray techniques enables the quantification of phases at multiple length scales to reveal their role regarding charge recombination in a highly efficient solution processed small molecule system 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh₂)₂) . A quantitative (linear) relationship between the average composition variations and the device fill-factor is observed. The results establish the complex interrelationship between average phase purity, domain size, and structural order and highlight the requirement of achieving sufficient phase purities to diminish bimolecular and geminate recombination in solution processed small molecule solar cells.

Resonant soft X-ray scattering reveals a correlation between average domain purity and device fill-factor for an efficient solution processed small molecule 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole):phenyl-C71-butyric acid methyl ester (p-DTS(FBTTh₂)₂:PC₇₁BM) system. The results show the requirement of an optimal combination of phase purities to diminish bimolecular as well as geminate recombination.

Light-soaking and hysteresis phenomena can reveal the bulk and interfacial polarization effects on photovoltaic processes in perovskite solar cells. In article number 1500279, Bin Hu and co-workers report the use of frequency-dependent impedance and time-dependent photoluminescence measurements to determine that bulk and interfacial polarizations are internally coupled in developing short-circuit current, open-circuit voltage, and fill factor through charge dissociation, transport, and collection in light-soaking and hysteresis phenomena.

A strategic approach for designing a novel hole-transporting material in perovskite hybrid solar cells and synthesizing a donor-p-acceptor polymeric hole conductor (TTB-TTQ) is reported by Taiho Park and co-workers in article number 1500471. A rough film surface with a fibril structure induced by (lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) salt and tert-butylpyridine (tBP) enhances the scattering effect and improves charge transport, resulting in a high efficiency (14.1%) with a high short-circuit current density (J_SC = 23.9 mA cm⁻²).

Interfacial energetics determines the performance of organic photovoltaic (OPV) cells based on a thin film of organic semiconductor blends. Here, an approach to modulating the “carrier selectivity” at the charge collecting interfaces and the consequent variations in the nongeminate charge carrier recombination dynamics in OPV devices are demonstrated. A ferroelectric blend interfacial layer composed of a solution-processable ferroelectric poly­mer and a wide bandgap semiconductor is introduced as a tunable electron selective layer in inverted OPV devices with non-Ohmic contact electrodes. The direct rendering of dipole alignment within the ferroelectric blend layer is found to increase the carrier selectivity of the charge collecting interfaces up to two orders of magnitude. Transient photovoltaic analyses reveal that the increase of carrier selectivity significantly reduces the diffusion and recombination among minority carriers in the vicinity of the electrodes, giving rise to the 85% increased charge carrier lifetime. Furthermore, the carrier-selective charge extraction leads to the constitution of the internal potential within the devices, even with energetically identical cathodes and anodes. With these carrier-selectivity-controlled interlayers, the devices based on various photoactive materials commonly display significant increments in the device performances, especially with the high fill factor of up to 0.76 under optimized conditions.

The carrier selectivity at charge collecting interfaces in organic solar cells is demonstrated through the utilization of ferroelectric–semiconductor blend interlayers. Transient electrical measurements reveal the elongated charge carrier lifetime within devices with highly charge-selective contacts, which is well correlated with the asymmetric development of charge carrier density profiles in the photo­active layers.

An 11.2% efficient kesterite solar cell is fabricated with a open circuit voltage (V_oc ) deficit of only 0.57 V, which is possible by employing the three-stage annealing process. The reduced V_oc deficit goes along with an increased minority carrier lifetime, low diode saturation current, and ideality factor, which are signatures of the semiconductor material with a low concentration of recombination centers.

Considering that a high compatibility at hybrid organic/inorganic interfaces can be achieved using polar and hydrophilic functionalities, this approach is used to improve inverted polymer solar cell performance by introducing nonionic phosphonate side chains (at 0%, 5%, 15%, and 30% substitution levels) into a series of isoindigo-based polymers (PIIGDT-Pn). This approach led to ≈20% improvement in power conversion efficiency compared to a nonmodified control polymer, via an increased short-circuit current (J _SC). This enhancement is believed to stem from reduced nongerminate recombination and improved charge carried extraction when the level of phosphonate substitution is optimized. These results are substantiated by a combination of detailed electrical measurements including space-charged limited current modeling, light intensity–dependent photocurrent (J _ph) analysis, and morphological studies (grazing-incidence wide-angle X-ray scattering and atomic force microscopy). This is the first practical report demonstrating the use of nonionic polar side chains to control charge carrier dynamics in an existing photovoltaic polymer structure. It is envisioned that this simple strategy may be applied to other material systems and yield new materials with the potential for even higher performance.

Nonionic phosphonate side chains are introduced into an isoindigo-based model polymer in order to induce a high compatibility at hybrid organic/inorganic interfaces, while controlling charge transport/collection and recombination dynamics in inverted polymer solar cells. This approach can bring ≈20% improvement in efficiency when compared to a nonmodified control analog.

As reported by Dong Chan Lim, Young Dok Kim, Shinuk Cho, and co-workers in article number 1500393, the efficiency of inverted bulk heterojunction solar cells is successfully enhanced by the incorporation of dodecanethiol monolayer-protected Au nanoclusters (Au:SR) based on sub-nanometer-sized Au38 cores. Solar cells with the Au:SR cluster exhibit 20% increased efficiency as a result of effective energy transfer based on the protoplasmonic fluorescence of Au:SR.

The addition of polystyrene (PS), a typical insulator, is empirically shown to increase the power conversion efficiencies (PCEs) of a solution-deposited bulk heterojunction (BHJ) molecular blend film used in solar cell fabrication: p-DTS(FBTTh₂)₂/PC₇₁BM. The performance is further improved by small quantities of diiodooctane (DIO), an established solvent additive. In this study, how the addition of PS and DIO affects the film formation of this bulk heterojunction blend film are probed via in situ monitoring of absorbance, thickness, and crystallinity. PS and DIO additives are shown to promote donor crystallite formation on different time scales and through different mechanisms. PS-containing films retain chlorobenzene solvent, extending evaporation time and promoting phase separation earlier in the casting process. This extended time is insufficient to attain the morphology for optimal PCE results before the film sets. Here is where the presence of DIO comes into play: its low vapor pressure further extends the time scale of film evolution and allows for crystalline rearrangement of the donor phase long after casting, ultimately leading to the best BHJ organization.

In situ measurement shows that polystyrene (PS) and diiodooctane (DIO) additives promote donor crystallite formation synergistically, on different time scales, and through different mechanisms. PS-rich films retain solvent, promoting phase separation early in the casting process. Meanwhile, the low vapor pressure of DIO extends the time scale of film evolution and allows for crystalline rearrangement of the donor phase after casting.

To alleviate the limitations of pure sulfide Cu₂ZnSnS₄ (CZTS) thin film, such as band gaps adjustment, antisite defects, secondary phase and microstructure, Cadmium is introduced into CZTS thin film to replace Zn partially to form Cu₂Zn_1−xCd_xSnS₄ (CZCTS) thin film by low-cost sol–gel method. It is demonstrated that the band gaps and crystal structure of CZCTS thin films are affected by the change in Zn/Cd ratio. In addition, the ZnS secondary phase can be decreased and the grain sizes can be improved to some degree by partial replacement of Zn with Cd in CZCTS thin film. The power conversion efficiency of CZTS solar cell device is enhanced significantly from 5.30% to 9.24% (active area efficiency 9.82%) with appropriate ratio of Zn/Cd. The variation of device parameter as a function of Zn/Cd ratio may be attributed to the change in electronic structure of the bulk CZCTS thin film (i.e., phase change from kesterite to stannite), which in turn affects the band alignment at the CZCTS/buffer interface and the charge separation at this interface.

9.2%-efficiency Cu₂ZnSnS₄ (CZTS)-based pure sulfide thin film solar cells are fabricated by replacing partial Zn with Cd via a low-cost sol–gel method. The appropriate incorporation of Cd into CZTS thin films can adjust band gaps, improve microstructure, and reduce the ZnS secondary phase in CZTS thin film, resulting in a significant improvement in efficiency.

The effects of a solvent additive, 1,8-diiodooctane (DIO), on both hole and electron transport are investigated in a state-of-the-art bulk-heterojunction (BHJ) system, namely PTB7:PC₇₁BM. For a polymer:fullerene weight ratio of 1:1.5, the electron mobility in the blend film increases by two orders of magnitude with the DIO concentration while almost no change is found in the hole mobility. For lower DIO concentrations, the electron mobility is suppressed because of large, but poorly connected PC₇₁BM domains. For higher concentrations of DIO, the electron mobility is improved progressively and the hole mobility becomes the limiting factor. Between 1 and 5 vol%, the electron and hole mobilities are balanced. Using the Gaussian disorder model (GDM), we found that the DIO concentration modifies fundamentally the average hopping distances of the electrons. In addition, there exist alternative donor–acceptor ratios to achieve optimized PTB7:PC₇₁BM based solar cells. It is demonstrated that the fullerene content of the BHJ film can be significantly reduced from 1:1.5 to 1:1 while the optimized performance can still be preserved.

The effect of the widely used solvent additive 1,8-diiodooctane (DIO) on the carrier transport in the PTB7:PC₇₁BM system is analyzed. The electron mobility in the blend film increases by two orders of magnitude with the DIO concentration while almost no change is found in the hole mobility. Its concentration fundamentally modifies the average hopping distances of electrons.

High-performance planar heterojunction perovskite (CH₃NH₃PbI₃) solar cell (PVSC) is demonstrated by utilizing CuSCN as a hole-transporting layer. Efficient hole-transport and hole-extraction at the CuSCN/CH₃NH₃PbI₃ interface facilitate the PVSCs to reach 16% power conversion efficiency (PCE). In addition, excellent transparency of CuSCN enables high-performance semitransparent PVSC (10% PCE and 25% average visible transmittance) to be realized.