The voltage loss in organic solar cells

<正>The power conversion efficiency(PCE) for donor–accepto bulk-heterojunction organic solar cells(OSCs) has reached～20%^[1–3], approaching that for inorganic solar cells, due to the development of key photoactive materials^[4–14]. The short-circuit current density(J_sc) and the fill factor(FF) fo state-of-the-art OSCs are already close to the thermodynam ic upper limit predicted by Shockley-Queisser theory^[15], but the open-circuit voltage(V_oc<...     

Cascade-type excitation energy relay in organic thin-film solar cells

We investigated small-molecule-based organic photovoltaic (PV) cells with three different electron-donating material layers, two thiophene/phenylene co-oligomers [α,ω-bis(biphenyl-4-yl)terthiophene (BP3T) and α,ω-diphenyl sexithiophene (P6T)] and copper phthalocyanine (CuPc), and one electron-accepting material layer (C₆₀). A cascade-type energy relay between the three donor layers occurred, and the incident photon-current conversion efficiency improved in the blue light region, where CuPc has very low optical absorption. An increase in P6T photoluminescence intensity in a two-layer sample (BP3T/P6T) on quartz confirmed interlayer excitation transfer (ET) from BP3T and P6T. The bulk heterojunction architecture in our interlayer-ET-based organic PV cell was effective. Moreover, P6T appeared to have a relatively long exciton diffusion length of several tens of nanometers.     

Saturation of photovoltage and photocurrent in p-n junction solar cells

Expressions for the photovoltage and photocurrent of a strongly illuminated p-n junction solar cell are derived by solving the ambipolar diffusion equation. A complete boundary condition is derived for the junction, which is valid for all levels of injection. In the open-circuit case, results are in agreement with those given by earlier theories, while in the short-circuit case, the current is found to saturate at the ratio of the diffusion potential to the internal resistance. Results are used to explain the experimental results of earlier workers.     

Fine structural tuning of diketopyrrolopyrrole-cored donor materials for small molecule-fullerene organic solar cells: A theoretical study

Two novel strategies to tune diketopyrrolopyrrole and its derivatives by lactam-lactim and alkoxy-thioalkoxy exchange were theoretically presented for improving the efficiency of bulk heterojunction solar cells. The structural tunings could synergistically reduce the HOMO level and the energy gap of donors due to the disrupted aromaticity of the linked pyrrole derivatives. Compared with the parent molecule, the new designed donors not only create a more red-shift of absorption spectrum but also show better hole transport rates, larger fill factor, higher open circuit voltage and more favorable solar cell efficiency. Moreover, the arrangements at the interface and the optical properties of the donor-PCBM complexes were computationally investigated to get insight into the absorption of charge transfer states. Consequently, the strategies are judicious approaches to enhance their intrinsic properties of donors and can be used for further improving the performance of other DPP-based molecules in bulk heterojunction solar cells.     

受体浓度影响体异质结有机太阳能电池复合损耗研究

制备基于P3HT:PCBM的复合体异质结太阳能电池 ,研究受体浓度对载流子复合特性的影响。测试结果表明, 受体浓度影响器件的电荷收集和复合损耗, 从而直接影响体异质结有机太阳能电池的光电性能; 光生电流随偏 置电压的增加而下降,光生电流下降趋势反映了载流子的复合损耗特性。理论分析进一步 表明,给体中电子与受体中 电子的密度比与受体浓度有直接关系,受体浓度改变双分子复合常数的大小,从而影响载流 子的复合特性。     

Enhanced absorbance and electron collection in inverted organic solar cells: Optical admittance and transient photocurrent analyses

Optical admittance analysis reveals that light absorption in inverted organic solar cells (OSCs), based on the same polymer blend layer of regio-regular poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM), is always greater than their regular geometry OSCs fabricated using an ITO/poly(3,4-ethylene dioxythiophene):(polystyrene sulfuric acid) anode. Transient photocurrent measurements elucidate that interfacial exciton dissociation at the cathode interfaces of Al-modified ITO/PCBM (inverted cell) and Al/PCBM (regular cell) is not equivalent. It is shown that the reverse configuration allows improving the absorbance of the cell, favoring charge collection at cathode/PCBM interface and also possessing a dawdling degradation behavior as compared to a control regular OSC in the accelerated aging test.     

A two-dimension-conjugated small molecule for efficient ternary organic solar cells

Ternary organic solar cells (OSCs) are burgeoning as one of the effective strategies to achieve high power conversion efficiencies (PCEs) by incorporating a third component with a complementary absorption into the binary blends. In this study, we presented a new two-dimension-conjugated small molecule denoted by DR3TBDTTVT, which alone gave rise to a best PCE of 5.71% with acceptor PC₇₁BM as active layer. Given the complementary absorption with PTB7-Th, DR3TBDTTVT was doped into (PTB7-Th:PC₇₁BM)-based binary blends, and ternary OSCs were developed. The ternary OSCs with 10 wt% of DR3TBDTTVT displayed improved hole-mobility, reduced device resistance and better phase separation of active layer, thus leading to an impressive PCE of 7.77% with open-circuit voltage of 0.77 V, short-circuit density of 14.52 mA cm⁻² and fill factor of 70.3%. Ternary OSCs well make up for the light-harvesting insufficiency of binary OSCs, and this research provides a new material for the improvement of PCEs for single-junction OSCs.     

Series resistance in organic bulk-heterojunction solar devices: Modulating carrier transport with fullerene electron traps

Series resistance is one of the key parameters affecting the performance of organic photovoltaic devices. Several electronic mechanisms arising from different structures within the solar cell can contribute to increasing it. We focus on the series resistance origin by altering the acceptor transport properties trough the incorporation of fullerene traps located at energies below the transporting electron levels. Indene-C₆₀ bisadduct as acceptor molecule blended with poly(3-hexylthiophene) forms the active layer in which small amounts of [6,6]-phenyl-C₆₁-butyric acid methyl ester have been added as trapping sites. A complete analysis of the impedance response has allowed identifying bulk transport resistive circuit elements in the high-frequency part of the spectra. Series resistance is observed to be dependent on the concentration of fullerene traps, thus indicating a connection between bulk transport processes and resistive elements. By comparing different contacts it has been discarded that outer cathode interfaces influence the series resistance experimentally extracted from impedance spectroscopy.     

Spectroscopic ellipsometry characterization of vacuum-deposited organic films for the application in organic solar cells

Variable angle spectroscopic ellipsometry (VASE) in the wavelength range from 245 to 1680 nm has been applied to determine the optical properties of the recently developed electron donor α,ω-bis-dicyanovinylene-sexithiophene (DCV6T), an efficient absorber material in organic solar cells (OSCs). To ensure uniqueness of the evaluation results interference enhanced substrates are used and comparison to simple native silicon substrates is presented. Similar as applied in OSC, DCV6T was deposited both as a pure single layer as well as in a mixed heterojunction with C60. For both cases, the in-plane refractive indices and extinction coefficients were higher than the out-of-plane ones, revealing that the DCV6T molecules in the films are preferentially horizontally oriented. This rough indication was further quantified by the so called molecular orientation parameter. Moreover, it is shown that annealing initiates molecular reorganization of the films, which leads to a higher birefringence and more defined horizontal orientation in the single layer. However, in the mixed layer annealing seems to reduce anisotropy. These effects and the consequences for the performance of organic solar cells are discussed.     

Ternary organic solar cells based on non-fullerene acceptors: A review

Organic solar cells (OSCs) have reached their second golden age in recent two years with a boosted number of publications. Non-fullerene acceptor (NFA) materials have become a rising star in the field which are widely applied in organic solar cells because of their excellent optoelectronic properties, such as strong light-harvesting ability and tunable energy level. Unlike the low synthetic flexibility and high production cost of fullerene materials, NFAs exhibit flexible structures, and relatively low fabrication costs. Recently, the ternary strategy has become another hot research topic in the field, which introduces a third component into the binary host system for OSCs. The application of a ternary strategy can break the limits of light absorption brought by the host system, improve the morphology and energy level alignment for the active layer and thus improved the efficiency of organic solar cell devices. Benefiting from the advancement in both NFA and ternary strategy, the power conversion efficiency (PCE) of organic solar cell has exceeded over 17.5% to date. A comprehensive review of the recent progress in NFA based ternary OSCs (TOSCs) is needed in the field. Herein, this review mainly focuses on recent research on ternary organic solar cells using NFA materials during the last two years. Firstly, device physics and frequently used active materials in NFA based TOSCs are summarized and discussed. Then, the recent reported high-performance NFA based TOSCs are reviewed. Finally, the outlook and future research direction in the field are proposed. This review aims to provide an insight into NFA based TOSCs and help researchers to explore the full potential of OSCs.     

Boundary condition model for the simulation of organic solar cells

Organic solar cells (OSCs) are promising photovoltaic devices to convert solar energy into electrical energy. Their many advantages such as lightweight, flexibility and low manufacturing costs are intrinsic to the organic/polymeric technology. However, because the performance of OSCs is still not competitive with inorganic solar cells, there is urgent need to improve the device performance using better designs, technologies and models. In this work, we focus on developing an accurate physics-based model that relates the charge carrier density at the metal-organic boundaries to the current density in OSCs. This analysis is based on our previous studies on single-carrier and bipolar diodes. The model for the boundary condition of the charge carrier density at the interfaces of OSCs follows a power-law function with the current density, both in dark and under illumination. Simulated current-voltage characteristics are verified with experimental results. The numerical simulations of the current-voltage characteristics of OSCs consider well-established models for the main physical and optical processes that take place in the device: light absorption and generation of excitons, dissociation of excitons into free charge carriers, charge transport, recombination and injection-extraction of free carriers. Our analysis provides important insights on the influence of the metal-organic interfaces on the overall performance of OSCs. The model is also used to explain the anomalous S-shape current-voltage curves found in some experimental data.     

Pinning energies of organic semiconductors in high-efficiency organic solar cells

With the emergence of new materials for high-efficiency organic solar cells(OSCs), understanding and finetuning the interface energetics become increasingly important. Precise determination of the so-called pinning energies, one of the critical characteristics of the material to predict the energy level alignment(ELA) at either electrode/organic or organic/organic interfaces, are urgently needed for the new materials. Here, pinning energies of a wide variety of newly developed donors and nonfull...     

Ternary mixing: A simple method to tailor the morphology of organic solar cells

We present a detailed study of the effects of ternary mixing on blend morphology, charge carrier mobility and organic solar cell performance. We investigate ternaries consisting of regio random poly(3-hexylthiophene) (P3HT), regio regular P3HT and soluble fullerene derivative, PCBM. By means of absorption, photoluminescence, atomic force microscopy and X-ray diffraction, we demonstrate that the structure of ternary films consists of crystallites of regular P3HT embedded into a random polymer matrix acting as a soft scaffolding where PCBM can only form nanoscale aggregates but cannot grow the detrimental micron-sized structures often observed in the conventional regular P3HT:PCBM case upon annealing. The ternary films exhibit higher degree of crystallinity than the conventional blends, but with smaller crystallite sizes. Moreover, we show that the addition of the random polymer chains does not prevent good charge carrier transport for regio random P3HT concentrations up to 50% of the total polymer content. Finally, we prove that solar cells based on the ternary systems have a similar short circuit current than the conventional binary, but improved open circuit current (by 100 mV), which leads to an overall enhancement of power conversion efficiency.     

Stability enhancement of normal-geometry organic solar cells in a highly damp condition: A study on the effect of top electrodes

We present our efforts to enhance the stability of normal-geometry organic solar cells (n-OSCs), which are generally considered inferior to their inverted-geometry counterparts in terms of stability. Upon the identification of the vulnerability of top electrode/buffer layer interfaces under a humid environment, various top electrode combinations are assessed under an extremely damp condition (27 °C, 90%) for n-OSCs based on a bulk-heterojunction of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C70-butyric acid methylester (PCBM70). Based on the experimental results, we propose an Al/Cu bilayer top electrode and demonstrate a 30-fold enhancement in the T₈₀-lifetime values. Our study reveals that the enhanced lifetime with an Al/Cu bilayer electrode can be attributed to its water vapor transmission rate (WVTR), which is significantly lower than that of the Al electrodes typically used in conventional organic solar cells. The enhanced normal OSCs yielded stability comparable to that of the previously reported inverted OSCs.     

Aging process of PEDOT:PSS dispersion and robust recovery of aged PEDOT:PSS as a hole transport layer for organic solar cells

Conducting p-type polymer of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely used for organic optoelectronics, particularly as a hole transport layer for organic solar cells. While the aged PEDOT:PSS dispersion impacts device performance, the aging of PEDOT:PSS dispersion have not been well investigated. Moreover, the recovery process of aged (two-year-old) PEDOT:PSS dispersion has not been demonstrated yet. Herein, it is found that aqueous PEDOT:PSS dispersion undergoes extensive phase separation during the aging process, resulting in both nanoscale and macroscale hydrophobic PEDOT-rich agglomerates. When the aged PEDOT:PSS thin film is integrated into P3HT:PCBM organic solar cells, the PEDOT-rich agglomerates trap the photogenerated holes at the PEDOT:PSS/P3HT interface, resulting in poor extraction efficiency in organic solar cells. To recover a hole transport functionality from aged PEDOT:PSS, three different solvents such as isopropyl alcohol (C₃H₇OH), ethanol (C₂H₅OH) and methanol (CH₃OH) are investigated. Among them, it is found that isopropyl alcohol (IPA) yielded very uniform PEDOT:PSS thin film layer. This is because hydrophobic functional groups of IPA solvent facilitated the preferential solvation of phase separated hydrophobic PEDOT-rich agglomerates. However, when non-optimal concentration of IPA solvents was added into the aged PEDOT:PSS dispersion, the size of PEDOT-rich agglomerates was adversely enlarged. When organic solar cells were fabricated using more than a two-year-old PEDOT:PSS that was treated with IPA solvent, the resulting device performance of organic solar cells was fully recovered and became comparable or better than that of organic solar cells fabricated with fresh PEDOT:PSS.     

On the transient reponse of solar cells to laser-beam illumination

Expressions for the decay of the photocurrent induced in a solar cell by a very short pulse of a finely focused laser beam are obtained, both for thick cells and thin cells with or without back surface field.     

A loss mechanism in MIS solar cells associated with the creation of energetic carriers

The scattering of energetic carriers generated by the absorption of short wavelength light (2500–7000 Å) is examined. Such carriers may undergo one or more scattering events before reaching the band edge. Thus they are able to surmount significant potential barriers and to contribute to the majority carrier current in MS and MIS solar cells.     

Investigating the reason for high FF from ternary organic solar cells

Organic solar cells (OSCs) have attracted huge attention because of their unique merits[1-3].In last few years,thanks to the design of new materials and device engineering,the power conversion efficiencies (PCEs) of OSCs have surpassed 18％[4-8].The PM6:Y6 cells are efficient binary cells,offering high PCEs over 16％[9-11].The high performance originates from the efficient free charge generation and the ground state dipole field at the donor-acceptor interface that pro-motes the exciton dissociation[12].To further boost the per-formance of PM6:Y6 cells,ternary architectures were adop-ted.Significant improvements in short-circuit current density(Jsc) and open-circuit voltage (Voc) were realized.However,most of the ternary cells still suffer from low fill factor (FF) (gen-erally ＜78％) (Table S1).The FF is generally determined by the competition between recombination and extraction of charge carriers[13-15].The interfacial electronic structures have nonnegligible impacts on charge transport in OSCs,also influ-encing the FF[16,17].Previously,Bao et al.demonstrated that the energy of positive integer charge transfer (ICT) states(EICT+) of PM6 is equal to the energy of negative ICT states(EICT-) of Y6,leading to no potential step at the PM6:Y6 inter-face and an ideal binary host system[18].To avoid the forma-tion of potential step in the ternary system,the EICT-of the second acceptor should be lower than (or equal to) EICT+ of the donor,thus suppressing the ICT trap-assisted recombina-tion[1].In this work,we carefully incorporate a second accept-or EH-IDTBR into the host PM6:Y6 blend (Fig.1(a)).From the ultraviolet photoelectron spectroscopy (UPS)-derived energy levels (Fig.1(b)),the negative ICT states of EH-IDTBR (EICT-=4.25 eV) is smaller than the positive ICT states of PM6 (EICr+ =4.5 eV),suggesting no ground state charge transfer at the PM6:EH-IDTBR interface,thus avoiding interfacial ICT trap-assisted recombination.     

Numerical study on the short-circuit current in single layer organic solar cells with Schottkey contacts

The effects of the cathode operation voltage,the absorption coefficient,the carrier mobility,the temperature and the thickness of organic active layer on the short-circuit current density of single layer organic solar cells(OSCs) with Schottkey contacts are numerically studied.Quantitative dependences of the short-circuit current density on the respective parameters are obtained.The results show that a larger operation-voltage difference between the anode and the cathode,a higher carrier mobility as well as...     

In-situ investigation of interfacial effects on charge accumulation and extraction in organic solar cells based on transient photocurrent studies

In organic solar cells, the interfacial and bulk photovoltaic processes are typically coupled based on charge transport and accumulation. In this article, we demonstrated that the in situ transient photocurrent measurements can be a powerful approach to separately investigate the interfacial effects on interfacial and bulk photovoltaic process. Based on this method, the effects of interfacial dipoles on charge extraction, accumulation, and recombination are solely studied by comparing Ca and Al devices with standard architecture of ITO/PEDOT/P3HT:PCBM/cathode. We observe that stronger interfacial dipoles can significantly decrease the charge extraction time and consequently increase the charge extraction efficiency. More importantly, stronger interfacial dipoles can also decrease the charge accumulation within the bulk photovoltaic layer. Furthermore, our experimental results indicate that the bulk-accumulated charges can act as recombination centers under device-operating condition, resulting in the recombination loss in photogenerated carriers. Clearly, our studies of transient photocurrents elucidated the charge extraction, accumulation, and recombination in OSCs.